Gardening with Rose
Admiral Biscuit
I’d thought that their backyard was the perfect place to practice going clothesless, but as I stood in the doorway I started to have second thoughts. It wasn’t as secluded as I’d originally thought; across the back fence I could see a stallion in the street pulling a wagon, and he hadn’t turned to look at me yet but it was only a matter of time. And of course there would be pegasi flying overhead, although that didn’t matter. Surely some of them had seen me before in my backyard or when I was bathing in the river and it hadn’t really bothered me then. I was just trying to rationalize.
Lily and Daisy were also staring. It was probably just curiosity. They didn’t faint on the spot or gallop off in terror.
I followed Rose over to a row, and we started working. Once I’d crouched down a bit to help her, the other two sort of lost interest in me and went back to their blooms.
We worked in silence at first, with Lily and Daisy occasionally studying me whenever I walked by with a loaded bucket. Lily was still tense; she kept flicking her tail and one ear was always pointed at me, no matter what she was doing. Daisy focused more on her tasks with only occasional glances in my direction.
I’d never noticed before, but while Rose and Lily exclusively used their tools with mouth and hoof, Daisy occasionally wrapped things in a green aura of magic even though she didn’t have a horn. That was something I’d never seen another earth pony do; that was something that just unicorns could do, or so I’d thought.
Obviously, Rose and Lily knew about it—how would they not? And even if it was rude to ask, I still wanted to know. “Rose? How come Daisy’s using magic?”
“She’s half unicorn,” Rose whispered back. “That’s why she’s kind of scrawny, too.”
“Technically, I’m three-quarters,” Daisy said. I hadn’t realized that she was close enough to overhear us.
I set my bucket down. This was interesting. “How does that work?”
“Well, when two ponies love each other very much—“
“Not that. I know how sex works.”
“Um.” She glanced over at Rose, who was blushing at the mention of sex. “Well, I don’t know exactly, other than it’s genetics and chromosomes. Dominant and recessive, just like plants.
“I had some magic surges when I was a filly, and most of the time it was making plants grow and stuff, but sometimes I did horn magic, so my Mom sort of helped teach me and encourage me to use telekinesis. Since almost everypony else in my family could, I didn’t think it was that odd, even though I didn’t have a horn. I do sort of have a little knot on my forehead, though.” Her ears drooped. “A lot of ponies don’t like it, and I guess it takes a while to get used to seeing. I don’t do it much in town.”
“Ponies don’t like the idea of unicorns growing plants,” Rose added. “When we’re all alone though, nopony knows.”
“The pony caste system at work,” I muttered.
“Huh? What’s that?”
“Never mind.”
Daisy’s ears perked back up. “Since we’re asking personal questions, how come your flanks and belly are kinda pale and the rest of you is more dun colored? Are humans piebald? Is that why you wear pants all the time, to hide it?”
“It’s a suntan,” I said. “My skin turns darker when it’s exposed to the sun, but if it’s under clothes that doesn’t happen.”
“Is that like a sunburn?”
“Not exactly, but I can get those, too.” In fact, there was a chance that that would happen, but I was going to take that chance. “This doesn’t hurt like a sunburn does. People that live in really sunny places on Earth have naturally darker skin; those of us that don’t have dark skin get a suntan, and only if we stay out too long, a sunburn.”
•••
By lunchtime, Daisy had warmed up to me at least, and I’d mostly forgotten that I was naked. With each passing minute, it felt more natural, and I didn’t worry any more about ponies going by on the street looking over and seeing me. Maybe if there had been a crowd gathered at the fence, I would have sung a different tune, but there wasn’t.
In fact, it was feeling right, like I was some kind of forest nymph returning to my natural state. Or maybe that was just the onset of heatstroke.
In the back of my mind I knew that there was still going to be a mental fight when it came time to leave the relative safety and seclusion of the backyard, but that was a problem for later. For now, I’d get used to this, and that would make the next steps that much easier.
A shadow crossed over the garden—a low-flying pegasus. I looked up just in time to see her circle back around and swoop past in the other direction, back towards town. Was that coincidence, or had she wanted to get a second look at me?
I guess I’d find out if a bunch of her friends suddenly showed up to gawk.
“Sam?”
“Sorry.” I turned my attention back to Rose. “Just saw a pegasus fly overhead and I was trying to remember if I knew her.” That wasn’t exactly true, but it was a reasonable excuse. “Kind of a medium blue with a lighter blue mane and tail.”
“That’s a lot of ponies,” Rose said. “Did you see her cutie mark?”
“Not all that well. I’m sure I’ve seen her around town, though.”
•••
It was a market day, so after we’d eaten, Daisy hitched herself up to their market wagon and the rest of us loaded it for her, and then she and Lily left for the market.
Rose and I should have gone back to work. Even though neither of us wanted to say it, I was sure I’d slowed them down just by being there and being a distraction.
“I never knew that about Daisy,” I said. “You never told me.”
“She’s kind of shy about it,” Rose told me. “I’m surprised she said anything; usually she just puts her head down and ignores the conversation.
“It must have been hard for her growing up to be a half-breed. Unicorns wouldn’t want to have anything to do with her, ‘cause she didn’t have a horn, and earth ponies would think she was too skinny and weak and wouldn’t like her casting magic.”
I thought I knew exactly why Daisy had warmed up to me, but I kept my mouth shut. Rose might have made the connection in her mind, too, but she didn’t say anything.
Before we could brood, I changed the subject. “Since they’re at market today, you’re supposed to cook dinner, right?”
Rose nodded. “But there’s still plenty of time to work in the flowerbeds.”
“I’ve got an idea I think that they’ll like. Remember that stir-fry I made for you? What if I make that? Have you got enough vegetables?”
“We don’t have bright peppers, only the green ones,” she said. “Does it matter what kind of vegetables you put in?”
“Not really.” In a stir-fry, the vegetables all tended to get along with each other. “When it’s time to start making it, show me where all the food is. You’ve got cooking oil and a big pan and a knife, right?”
“Of course. What kind of kitchen doesn’t?”
“There are people on Earth who only have a pot and a bowl and a spoon in their kitchen,” I said. “You’d be surprised how many boxed meals you can make with just that.” I got up and walked toward the backyard, with Rose following, and we settled into place along the flowerbeds.
“How are you feeling?” Rose asked. “If—you can put your shorts back on if you’re uncomfortable.”
“I’m fine.” And that was the truth. “This is really nice, honestly. I don’t know why I didn’t think of it sooner. And it’s lots better than cleaning outhouses with Honey Dipper.”
Rose stuck out her tongue. “Anything would be. She sells really good fertilizer, though.”
I hadn’t thought about that, even though she’d told me that was what she did with it. I reflected back on the stinking pits and barrels, and the less smelly rows of composting waste, and then the rich loam that made up the flowerbeds. Maybe one day, some of the fruits of my labor would be added to the soil.
“She said that ponies in Canterlot and Manehattan don’t want to see manure wagons during the day, and some ponies in Ponyville don’t want to, either.”
“‘Cause they don’t want to think about where their food comes from,” Rose said. “All of us farmers know, though.”
“Do any farmers make their own compost?”
“Sure, especially if they’ve got animals.” She leaned down to a blood-red rose and snipped the stem off. “Not in town, though; nopony wants that.”
•••
Squash and radishes and carrots provided plenty of color for the stir-fry. Since it had been so popular with Rose, I made plenty, alternating between cutting and frying. There wasn’t really a big enough pan to cook them all at once, so I put the cooked veggies in a casserole dish in the oven to keep them warm.
I sliced the vegetables as thin as I could manage, which took a little bit of extra time but was totally worth it.
Being out of the sun was nice, although the kitchen got hot, even with the windows open. The stove had very little insulation on its outside, which I suppose was typical of wood stoves.
Aside from the heat, I had to be careful of the hot oil. Rose offered me an apron, which was more of a bib on me, so I turned it down. Still, I had plenty of practice from cooking at home, and only got hit with a few drops of hot oil.
When the four of us sat down to dinner, Daisy and Lily looked at the vegetables admiringly. “You didn’t have to have somepony come and cut them,” Lily said. “Thicker vegetables would have been fine.”
“Sam cut them,” Rose said.
I half expected Lily to push her plate away, but she didn’t. “Really?”
I nodded.
“That must have taken forever.”
“It didn’t take all that long.” I was no expert chef, but even so I could chop up a vegetable in under a minute.
“It’s good.” Daisy was admiring her dinner the proper way, by eating it. “It’s really good. Rose, did you have Sam cook dinner for us?”
“We were behind on cutting flowers, and I can do it better than Sam can.”
“I’m surprised you don’t have a knife cutie mark.”
“Humans don’t get cutie marks,” I reminded her.
“No coat, no tail, no cutie mark, big udders . . . I’m glad I’m a pony.”
Daisy frowned. “That was rude, Lily. Sam can’t help being born a human. Maybe if she got to choose, she’d be something else, but she’s not and so she has to make the best of what she is.”
“I don’t want to argue at the dinner table.”
“You shouldn’t have brought it up, then.” Daisy pounded her hoof on the table, rattling the dishes. “Why not appreciate how many buckets of flowers she can carry at once or how she can cut the vegetables really thin instead of saying mean things about Rose’s—about our friend.”
“I wasn’t trying to be rude, you’re the one who’s got burrs in her tail.” Lily stuck her nose in her bowl and took a mouthful of stir fry. Once she’d finished chewing, she looked in my direction. “We get along fine, don’t we?”
“Yeah, of course we do.” That wasn’t true, but if it stopped the argument, it was worth saying.
I turned to Daisy. “If it makes you feel any better, I don’t know if I’d want to be a pony. I like having hands, and I don’t know how I’d feel about having a tail or a coat. Seems like it would be hot all the time and take forever to dry.”
“I’m sorry.” Daisy hung her head. “I was being rude. I shouldn’t have said anything.”
“Your heart was in the right place. And that’s what matters. There’s plenty more stir-fry, so you can have seconds if you want.”
One word for tomorrow: Sunburn.
9723113
The ponies will likely freak if she blisters...
That wound being a lot more casual than I expected. It was also very interesting that you explored pony hybrids. And who knows, maybe Sam will start working on her all-over tan from now on.
Good thing Sam didnt try sunbathing face down. Wake up next day with a treasure map on her back.
Nothing summarizes the outdoor nudist experience like this.
I really love the way Daisy and Sam discover common ground.
That said, I lost track of speaker identity at the dinner table and had to rad it thrice. Add a few more 'x said' maybe?
Maybe I'm just thick though.
Oh and yes, sunburn. So much sunburn. Ponies will think she's half chameleon, (possibly a venomous one, given the warning colour) she'll even shed her old skin like a proper reptile.
Wow no, Daisy was actually right there and not rude. Lily was far more unpleasant.
Actually plant's are usually more complex. Most animals (on earth at least Ponies might be different) have 2 copies of their chromosomes so a maximum of 2 versions of each gene to interact (more or less some tissues which produce lots of proteins such as liver have more and sometimes there can be multiple copies of the same gene on one chromosome). However, about 50% of plants have more than 2 copies, in some cases 20 or even more so they can have lots more different versions of the same gene interacting, it makes things much more complex than just Dominant and Recessive.
The issue is that if you have an odd number of copies they don't normally separate equally during sexual reproduction so your offspring have varying numbers of copies which is normally fatal or at least very harmful, plants can avoid it by reproducing asexually where as most animals can't.
Of course even animals aren't quite as simple as most traits are controlled by multiple gene's. There's not a single 'brown hair' gene even though a single gene might code for the pigment that makes that colour other genes will effect the level of chemicals needed to produce that pigment or control how strongly the first gene is turned on leading to a wide range of different colours.
9723927
You didn't steal that from Wiki, did ya? (You're not wrong, that's exactly how strawberry genes operate.)
Ooh, reference to a blink-and-you'll-miss-it thing from Look Before You Sleep.
Another chapter so soon? What madness is this!?
9723000
I don't know. the way the horn changes color is kinda weird.
I like the bonding with daisy and Sam.
However, i'm not sure how well the genetics work out. Wouldn't we expect to see blending on the external phenotype with horns?
9724387
I mostly got it from three years earning a degree in Genetics.
This reminds me that I need to get some poop for the garden this autumn.
9725940
Dried chicken manure is usually the best if you can get it. Reeks to high heaven but you won't argue with the results.
9724914 Depends on issues of genetic dominance and penetrance, also how multi-genic the initiation pathway is during development.
If just one key transcription factor initiates the program, and all the ponies came from a common root ancestry, then you'd expect to see mainly full horns on any pony which inherited the gene.
If it takes two genes to activate it, a pair of Earth pony parents could each carry one and not express, but 1/4 of their foals would be unicorns (1/2 from mother x 1/2 from father). Same thing might be the case for Pegasus wings. So you could have Earth ponies each carrying the partial genotype for both unicorns and pegasi, and have offspring of each type. Now, to get a non-divine 'alicorn' phenotype, that would be 1/2 x 1/2 x 1/2 x 1/2 = 1/16. So even in ponies that were heterozygous for both unicorn and Pegasus genes, it'd be fairly uncommon. Even moreso if there were additional repressors carried in phenotypically Earth pony individuals. Then, you'd need to have at least one repressor allele to be a nonfunctional mutant (premature stop codon, loss of function due to altered promoter region, or missense mutation) to have any chance of getting an alicorn phenotype. In addition, repressor genes would lower the chances of hybrid Earth ponies having a unicorn or Pegasus foal.
I did a 'Cake Family Genetics' blog a long time ago to show how that could work.
9725953 Too many neighbors. I tend to stick with horse manure. Least revolting of the possibilities.
You also end up with all manner of weeds sprouting from seeds which mostly pass through undamaged, some of which I find uses for:
Velvet leaf, I discovered, has many uses. I've made flour from the toasted seeds (tastes a bit like roasted peanuts) and test batches of twine from the long stem fibers (it's very strong. So is the fiber of Rose Milkweed.) Then there's aramanth, which has very nutritious seeds which are produced in the millions from just a few dozen plants. Purslane, which is delightful in salads, especially with a little balsamic vinaigrette and feta cheese. And about half a dozen other edible weedy plants.
The tan lines bit was brilliant!
Well, Daisy just got more interesting. I've seen a few fics where a unicorn or pegasus is described a little bit bigger or stronger because they're part Earth pony but this is the first time I've heard of an Earth pony with a nubbins of a unicorn horn. Very interesting. images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/f/77c6869b-6b59-4cf5-9dde-e3564d5a06de/d6mjbqd-2d64331d-4132-449c-8e70-930770bd0778.jpg?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ1cm46YXBwOjdlMGQxODg5ODIyNjQzNzNhNWYwZDQxNWVhMGQyNmUwIiwiaXNzIjoidXJuOmFwcDo3ZTBkMTg4OTgyMjY0MzczYTVmMGQ0MTVlYTBkMjZlMCIsIm9iaiI6W1t7InBhdGgiOiJcL2ZcLzc3YzY4NjliLTZiNTktNGNmNS05ZGRlLWUzNTY0ZDVhMDZkZVwvZDZtamJxZC0yZDY0MzMxZC00MTMyLTQ0OWMtOGU3MC05MzA3NzBiZDA3NzguanBnIn1dXSwiYXVkIjpbInVybjpzZXJ2aWNlOmZpbGUuZG93bmxvYWQiXX0.M19k_fmM5xOVexTN7sKffHk-dRbwFevRXFiN4nKTK24 I'm glad Daisy and Sam got to bond a little bit. Great chapter as always and I look forward to reading the next one.
9725957
Yeah, i've worked out the genetics of how it could work pretty similarly, although not to quite that extent. My question is more on the expression side. Can you get unicorn magic without the horn, and can you partially express unicorn genes (which is what I assume is the case with Daisy)? It's mostly headcanon, but it's still a conversation to have.
9726050 Now that would all depend on how magic inheritance works... which our mundane science has absolutely no foundation for, since there is no magic in our universe.
It'd be like asking how X-Men work 'in reality' and why a single X gene triggers an endless variety of mutant powers with little to no similarity.
Some fictional things you just have to shrug off and hope they at least adhere to their own internal world rules.
9723113
Possibly in places that aren’t overly comfortable. But, Sam’s probably gotten enough sun exposure in her time in Equestria for it not to be severe.
9723296
As well they should . . .
I’ve gotten a bad enough sunburn to blister before. Do not recommend.
9723503
Well, as Sam keeps telling herself, she’s getting all worked up about nothing.
I’m kind of surprised more people don’t, honestly.
She might--either being more comfortable around her home or perhaps in time all over Equestria. At least until the winter, anyway; she’s not likely to want to go out in just boots, hat, and scarf.
9723548
I don’t know if it’s worse on the back or the front. Depends on what gets burned, I suppose.
9723637
It’s true. It’s weird and uncomfortable, and then once you’re past that, it feels like how things ought to be.
9723672
No, that’s fair. I got a bit of pre-reader pushback from that, too. I don’t like using dialogue tags when I can help it, but I do sometimes trim too many. Easy to know who’s saying what when you’re writing a section, less so when you’re reading it for the first time.
The good news is that at least a fair portion of her body is already well-tanned, and if Equestrian clothes let at least some UV through, she might already have a decent base tan. Natural fibers (which I assume the ponies use) don’t block as much UV as synthetics.
9723691
Yeah, Lily’s being a total btch. No question.
9723927
I would assume overall though, they still work in a dominant and recessive sort of way, just with a lot more complexity, right? At least, that’s what I remember learning about with that monk and his experiment with peas (his name is right on the tip of my tongue, and I don’t want to cheat and look it up . . .) Mendel, maybe? That sounds right.
Or am I completely off-base with this? I’ve always assumed that if you mix, say, pink and white flowers, you’ll wind up with offspring that are various shades of pink to white, but probably not blue ones or yellow ones . . . the Dame’s Rocket I’ve got growing in my front yard is like that. Purple to white flowers, and all shades in between.
9724518
Honestly got me thinking, that animation error did. And I realized that maybe there were earth ponies who could do casting magic, if their parents were unicorns. Further evidence that ponies could produce non-tribe offspring came with the Cake’s foals, and I just ran with it. It hasn’t appeared blatantly in any one of my other fics (except for a silly one-shot with the Flower Trio), but I seem to recall it’s also hinted at in OPP.
9724531
This is me wanting to get the story ready in time for the Bronycon Bookstore, which means that I need to have a finished draft by . . .
<looks at calendar>
. . . this coming Sunday.
Can’t promise that the online version will have all the chapters by then, though. But it’ll either be before Bronycon, or else I’ll not be selling this at the bookstore.
9724911
Does raise some interesting questions about the horn, though. Is it fur-colored--or fur-covered--so it matches? Is it just because Sweetie is white, and all unicorn horns are white without the fur cover? Maybe it matches because it’s made of keratin, same as the hair . . . in which case, a paint unicorn might have a multicolored horn.
9724914
Thanks!
I don’t know what that means, exactly. Do you mean that there should be other visible external characteristics to a mixed-breed pony? Because in that case, I think that there might be, although we don’t generally see them in the show, basically because in the early parts, the animators were lazy.
9725953
I’ve heard llama poop is also good, but never tried it.
9725957
In my totally not a biologist mechanic’s head, I sort of think of it working similarly to sex chromosomes in humans, in that a lot of stuff changes when one box or the other is ticked, and depending on the case there can be some crossover because reasons. I personally don’t think that normal ponies can have alicorn offspring, although I have no reason to expect that it wouldn’t work. Likewise, it’s basically canon that ponies and donkeys can get it on, since they have hinnies, and I’d assume that just like IRL horses, they can hook up with zebras, too. For the other, more magical races (like seaponies), I have no idea how it could work biologically.
Link plz.
9725969
Very useful!
I’ve long assumed that many earth ponies snack on the tasty weeds in their gardens, because why waste a free meal? Those dandilions might not be sellable at the market (well, maybe they are in Canterlot, where you can charge extra ‘cause they’re feral), but they’re perfectly acceptable to nom on while gardening.
Also related, I recently found out from a probably reliable source that horses can eat poison ivy with no ill effects, and also stinging nettle, which means that humans in Equestria might want to avoid the ‘spicy’ salads.
In fact, let’s assume that ponies name anything that is toxic to humans but not equines as ‘spicy’. “Oh, that’s my crop of spicy oak, and I’m growing spicy nettles next to it. . . .”
9726008
Thanks!
I mean, if it works for non-earth ponies with earth pony parents, why wouldn’t it work for the other tribes?
In my own headcanon, Bulk Biceps is part Earth Pony (hence his bulk).
I also think that Rarity is part Earth pony. Now, I know the show’s jossed it by revealing that Magnum is in fact a unicorn, but nevertheless, it’s a fact in the show that Rarity more often uses physical attacks rather than magical.
I’ve also considered that Cherry Berry might be part pegasus, due to her love for the sky. Now, we do know that she’s related to the Apple family--that’s show canon--but we also know that there are some pegasi in their family tree. . .
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9726050 9726268
I assume you can. I mean, obviously, given the case here. We do know from the movie that a unicorn can still cast with a broken horn, and if we accept the animation error in Look Before you Sleep, ponies can cast with no horn (and there may be other, similar animation errors).
And Alondro’s point is fair. There’s no reason why pony biology has to work like mundane biology . . . Estee wrote a story where when and how you did it and what direction you were facing might change what your foal was, and while that was a comedy, there’s no reason why that couldn’t be the case for ponies. As I said in a comment above to Alondro, my own headcanon as a mechanic is that it works sort of like human sex chromosomes--most of the time, you express one or the other, but sometimes there’s blending of both. In other words, I don’t imagine it working where all the offspring of a unicorn and an earth pony come out as varying mixes of earth pony or unicorn--generally, they’ll be one or the other. But sometimes, there will be edge cases.
I also think that there could be a bit of upbringing involved. Like, maybe if Daisy’s parents hadn’t encouraged what magical expression she could do (presumably only simple stuff, basic TK, light spells, and so forth), that’d be something that she just didn’t ever learn to do. I mean, this is probably a weak example, but I eat left-handed because I learned by mirroring my Mom . . . amusingly enough, that was something nobody in the family had noticed until my left hand was in a bandage and I couldn’t manage utensils with my right hand.
9726321 Not there, she hasn't.
I see a red moon rising...
9726392 You can actually eat stinging nettle if you boil it when it's young (older plants are gritty-textured), or make a cordial out of the leaves by soaking them in sugar solution.
Mr. Salad Fingers also found a use for nettles, though... notsureifwant...
9726426 Earth ponies can gain unicorn powers... by drinking unicorn blood.
Just like Voldemort!
Two updates?
So happy.
9726445
Well, that depends on how good pony clothes are at blocking UV rays. Of course, since she does have tan lines, they clearly have some effect.
And this is probably more than you need to know, but I can attest that that’s less pleasant than you’d think.
9726605
Yes, you can eat lots of things when they’re young and tender.
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And Pegasi learn English by eating the brains of an English teacher.
I want seconds, and thirds, please!
That Lily stuff was nice. I'm surprised the show never explored that aspect of pony genetics.
9726349
The Monk your thinking of is Gregor Mendel an early geneticist who formally discovered some of the rules of inheritance (of course animal and plant breeders had known then in general for millennia but he formally studied it and wrote it down (later in his life he was actually ashamed of his work and tried to destroy it but a gardener he had been working with kept a copy and it was revealed years later).
In general yes you have dominant and recessive genes but it can get more complex than that Gene variant A might be dominant to B but not C while B may be dominant to C. With more granular factors like height multiple gene's tend to work together so there might be a dozen genes effecting height each adding between 1 and 10 cm and total height depends on which ones you have.
9726634
That is, in fact, the method by which I came to acquire my sophisticated manner of articulation. Wot ho, chip chip cheerios.
9726268
I'd say it's close enough to get an understanding.
And I find it really hard to resist going down the rabbit hole of what those internal rules are and how well they work. It's just how i'm wired. I've never really seen X-men at all and I immediately started wondering why this is the case.
9726369
personally i think it's kind like keratin where it's actually a colored substance. paint unicorn, hmm ... I haven't considered that case. That's an interesting question.
I'm saying that while mixed parentage would definitely be a thing, it doesn't seem like it would really effect being a unicorn vs an earth pony. With some characteristics, the genes can both be expressed. Take pink flowers for example, in some cases it's both red and white flower genes being expressed at the same time. But with something like a unicorn, it seems like you're either a unicorn or an earth pony, not both. That being said, I think some characteristics can be associated with different tribes, like unicorns being more slender, kinda like how there's genes associated with geographic regions.
personally I don't really accept stuff like this, especially animation errors. Because then ponies can spontaneously grow/lose/detach body parts and well ... this.
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It'd be a pretty good idea for a one shot, but for headcanon not as much (at least for me).
The issue is that equestria has so much borrowed from our world. Does equestrian biology overwrite our biology in the cases where we already know stuff? Obviously there's stuff we don't know, but we do know a bunch of stuff already. And if you like the idea that there's been contact between earth and equestria at some point it makes perfect sense to use our knowledge of biology (although this is mostly for certain types of fics).
That's not a bad way to think of it, but there are some weak areas. Edge cases like that are exceedingly rare (i'm talking 0.003%) and they tend not to work as well (sterility is pretty common). So while I think it's very likely that cases like that exist, it'd be more of a medical condition than a regular pony. I can easily imagine an earth pony being born with an entirely nonfunctional horn or something like that.
Personally, I think the horn is essentially an organ required for unicorn magic. I did kinda assume this in my original comment (which is what sparked it) but the show does kinda back this up (it glows while their using magic). If it isn't the case, it means that there's more to being a unicorn than having the horn (i mean obviously the horn needs support structures but you know what I mean). which seems kinda weird to me.
9727800
Also, Diamond Mint literally losing her head at the Equestria Games.