Hello once more, fellow Bronies and TYPE-Lunatics! Aedan again, back with another MLP:FiM/Nasuverse crossover Theory, which will hereafter be called TYPE-Pony Theories! This new theory covers an idea I mentioned in passing during my Elements of Harmony theory (here's the link, if you're interested: The Elements of Harmony in Relation to The Nasuverse ): The idea of Changelings (y'know, those bug-pony-vampire... things) being the Equestrian equivalent of the True Ancestors that hail from one of Kinoko Nasu's oldest works, Tsukihime. To start things off, I will have to describe the True Ancestors in a simple, yet detailed summary of who they are, where they came from, what they were meant to do and how they would do so. Then we will examine the Changelings, specifically where they parallel the True Ancestors, where they do not, and why. With me so far? Good, then let's begin.
PLEASE TAKE NOTE: I will not be taking the comics into account here! I do not consider them canon, but rather an alternate reality with many differences. This is MY opinion, and if you have your own, that's just fine. Just don't give any flames, please.

In The Beginning...
The Earth tried to brainwash mankind. No, that's not a joke. In essence, the will of the planet, Gaia, became wary of one of its own species of inhabitants: humans. You see, Gaia is the collective will of the planet to survive, with nature as the dominant factor. To uphold this will, a defense mechanism called the Counter Force acted proportionally against threats that showed up. Like I mentioned in my Elements of Harmony post, the Counter Force could call up just enough power to roll a small pebble, or enough to submerge an entire continent a la Atlantis. It almost never acted without a medium, however, instead working through proxies in the World. The Counter Force of Gaia utilized what are called Phantasmal Species. These are beings that do not follow the usual laws of biology, even when mutations are taken into account. Of particular note here are the Transcendent Kind, where are essentially terminals of the planet's will split from Gaia proper and given physical form and independent volition, but still subservient to the planet. These beings, including Divine Spirits, Oni Kind, Dragon Kind, Elementals, and Faeries, are the main tools and mediums of Gaia's Counter Force, and Earth's front-line protectors from general threats.
As time went on, however, Gaia noticed that one species in particular was diverging from itself. This species was, as you may have already guessed, mankind. Humankind's will was simply a part of the Gaian system at first. However, humanity began to grow independent of the World, and their development began to diverge from the other species. As this happened, their collective will to avoid extinction, called Alaya, not only separated from Gaia, but grew more powerful, due to the fact that the more prolific a certain race is, the more powerful will their collective unconsciousness become. In a very short span of time, Alaya not only equaled Gaia in power, but had the potential to smother it outright. This, of course, frightened Gaia, to the point where it sent out a call for help from its "siblings," the Ultimate Ones.
Now, while I'll cover the Ultimate Ones more thoroughly in a future theory, I'll give a brief overview here. The Ultimate Ones are to the other celestial bodies what Gaia is to the earth -- but to a greater degree. You see, Gaia, while powerful, is formless and intangible, only able to act through mediums and proxies. The Ultimate Ones are essentially both incarnations of their respective planets and the planets' pinnacle lifeforms at the same time. Only a celestial body could understand the death of another celestial body, so it's understandable that Gaia would seek assistance from the Ultimate Ones, the planets incarnate. The one to actually respond, however, is the one that every Nasu fan will know by heart (and make bronies everywhere reading this facehoof and say, "Of bucking course, what else?"), the name sake of our beloved franchise, TYPE-Moon himself.

The Crimson Moon: A Nightmare By Any Other Name
Ah, TYPE-Moon. Where to begin on this guy? For one who has made such an impact on the Nasuverse's history, we know little about him. We do know that he was the first of the Ultimate Ones to respond to Gaia's plea, striking a bargain with the will of the planet: in exchange for creating a solution to Gaia's human problem, TYPE-Moon would be permitted to come to the Earth's surface and live there. While on Earth, he acted very much like a vagabond ronin, traversing the land and challenging random beings, gaining followers along the way, including now famous individuals Merem Solomon, Gransung Blackmore, and Trhvmn Ortenrosse. During this period, he became known as "Brunestud of the Crimson Moon." Tsukihime fans will find this name familiar, and there's a very good reason for this-- but that reason has no place in this theory, and will be covered elsewhere. Ultimately, he desired to create a kingdom of his own on earth, and ultimately rule the planet. In the end, Kischur Zelretch Schweinorg, the Second Magician, was the one to ultimately "kill" Brunestud of the Crimson Moon, leaving behind the defense force he crated on Earth's behalf: the True Ancestors.

The Real of the World: WARNING -- Contents and Results May Vary
The True Ancestors were arguably the pinnacle of life on Earth -- or they WOULD be, if they were entirely Earth-born. Physically, True Ancestors look like humans, but "human" was the last word you could describe them with. The only reason the True Ancestors had human forms was because Brunestud felt that those who would dominate and rule should physically resemble those that they would dominate. They were actually a new type of Fairy -- the same kind of spirit that made Excalibur -- engineered by and based on Crimson Moon himself. Even among Faeries, however, they are unique. Normally, Faeries are imperceptible to humans. However, a Fairy may grow to the point where humans could see them directly. When this happens, they are classified as Elementals -- essentially forces of nature given form, thought, and self-volition. Brunestud spared nothing in creating the True Ancestors, essentially cloning himself in the process of their creation.
The True Ancestors were the perfect answer for Gaia's problem with humans, it seemed. Not only could they control reality itself to a degree (a power called Marble Phantasm, akin to a Reality Marble but still within the laws of nature), they could control the minds of men via a power called the Mystic Eyes of Enchantment. Brunestud called their primary duty the "Real of the World"; in short, they were to return the earth to the state it was in before humans came along and screwed things up, like humans do. Since they were mediators and caretakers of the planet first, and enemies of humans second, the Counter Force of Alaya (Heroic Spirits and Counter Guardians) would not recognize them as threats, and thus take no action against them. It seemed too good to be true... and it was. As powerful and effective as the True Ancestors were, Crimson Moon and Gaia failed to take into account two things: 1) that since humans had no part in the creation of the True Ancestors, unlike other Elementals, the True Ancestors would inevitably fade away; and 2) since Crimson Moon was literally an alien from a different ecosystem on earth's soil, the True Ancestors, being all but clones of Crimson Moon, would be the same; and a certain flaw would be inherited from Brunestud: the Vampiric Impulse.
Every True Ancestor was born with a purely psychological craving for human blood -- their bodies had no need of it. This is markedly different from other "bloodsucking species" like Dead Apostles, who require blood for sustenance, lest they starve. Because this craving, dubbed the Vampiric Impulse, was purely psychological in nature, it could only be suppressed -- and such a base craving couldn't just be ignored. A True Ancestor would have to partition an enormous fraction of his own power into suppressing the Impulse, practically crippling the True Ancestors as a whole. Worse yet, those who inevitably succumbed to the Impulse became half-mindless Demon Lords, who, with their liberated blood lust, unfettered powers, and total madness, posed a threat to Earth, humanity, and themselves. In response to this turn of events, the remaining True Ancestors congregated and engineered the birth of the ultimate True Ancestor. This was the White Princess Arcueid Brunestud, a living superweapon against the Demon Lords and the Dead Apostles, humans-turned-vampires as a result of being drained of blood by Ancestors and Demon Lords. While a potential solution to the Demon Lords was found in Arcueid, another problem remained: As humans had no part in the creation of the True Ancestors, in a world where human perception mattered most, the True Ancestors ultimately dwindled away. In the present day, only Arcueid Brunestud remains, the Last True Ancestor, battling away against the remaining Dead Apostles and their aristocracy, the 27 Dead Apostle Ancestors.

And thus did the era of the True Ancestors end.

Changelings and True Ancestors: See the Family Resemblance?
So now, after so much exposition we get to the real meat of the theory: Changelings. How do they compare to the True Ancestors? The answer is quite well, actually. Both the similarities and differences we see matter heavily as to how they are connected.
Similarities
The similarities between the two are easy to make out, if one knows where to look. As with the True Ancestors to humans, Changelings seem to be a natural counter to ponies, in that they devour the most intrinsic and vital part of a healthy, functional pony -- emotions. For ponies, emotions are everything, even more so than they are for humans. It is as vital as blood to them; they just can't live without it. Joy, anger, love, hatred, it doesn't matter -- they feel and express these emotions more deeply and freely than any other race in our world or theirs. The Changelings take that away, reducing ponies to catatonia and susceptibility to suggestion. The Changelings weaken their targets and control them, just as the True Ancestors do. Both have traits of faeries in their nature - the True Ancestors being faeries in the literal sense, and the Changelings possessing many key traits of the Celtic Fair Folk, who were not to be crossed whatsoever. They also take the mimetic appearance of the True Ancestors to the logical extreme. True Ancestors, natural enemies and rulers of humans, take after them in general appearance. Changelings, natural predators and controllers of ponies, can assume the exact shape of any pony they directly see, in the blink of an eye. In a way, one could say that Changelings were deliberately made to subdue and weaken the pony races as a whole.
On another note, both the Changelings and the True Ancestors are (or rather, were) in decline; the only True Ancestors left in the modern era being Arcueid and Altrouge Brunestud, and the Changelings living as only starving, skin-and-bone husks ruled by animal hunger, and slowly dying out. As of the Season 6 finale, this has, of course, changed and I will cover why shortly.

The Difference
It is probably obvious to readers by now, but despite the many similarities between True Ancestors and Changelings, there are key differences, as well. These differences, physical forms not withstanding, are the most important part of this theory, and I think you may be surprised. Most differences are easy to spot: the Changelings do not need or desire blood, nor are they made with the help of an extraterrestrial entity (to the best of our knowledge, anyway.). A less obvious major difference is the reason for their respective decline. The True Ancestors died out mainly because of a combination of the Vampiric Impulse and a lack of support from literally half of the planet. Changelings were declining due to a distinct lack of food, largely through their own actions (or at least the actions of Chrysalis.). But it goes deeper than that.
Remember in my Elements of Harmony post that I postulated that in Equestria, the Counter Force exists in a similar state as on Earth: namely, the presences of a planetary will and a sentient group's collective unconsciousness. I referred to the Equestrian versions as E-Gaia (Equestria-Gaia) and Alaya-P (Alaya-Pony), respectively. While the two forces are similar to Earth's versions, their relationship is not. Gaia and Alaya are almost constantly at odds; that is the very reason the True Ancestors exist. E-Gaia and Alaya-P don't seem to clash too much, however. Why? The answer is that ponies and other sentient species are not a threat to the planet -- or at least, not anymore. Ponies, in particular, seem to be extremely respectful of the nature around them, and even aid in its functions. How can they be a threat if they actively benefit E-Gaia as a whole? This is where the Changelings are concerned -- and where the true source of their decline lies.
E-Gaia probably made the Changelings just when ponies and other races like Griffons, Minotaurs, Diamond Dogs and Yaks were on the rise, as a fail-safe and insurance, just in case the sentient races turned out to be a threat to the planet on the long run. They didn't, so what about the Changelings, which were fed mainly in the process of performing their required task? In a nutshell, they became unnecessary. E-Gaia didn't need them anymore, so the Counter Force didn't support them, and they were left to fend for themselves without any help. The planet no longer supported them, and the sentient races they were meant to subdue and feed off of avoided them like the plague. Harsh, right? Well, that's par for the course for planetary wills. They don't really have a concept of "good" or "evil." They only know "useful," "useless," and "threat." The Changelings fell into the second category, and thus were forced to strive for their own existence. This, combined with the self-centered obstinacy of their later queen, Chrysalis, nearly led to their extinction, which was only averted by Thorax's rebellion and subsequent revelation that Changelings can share their emotions and survive that way. Up until that point, the Changelings resisted the one concept that permeated the World in its entirety at the time: harmony (which was, ironically another direct result of E-Gaia's non-hostility and cooperation with Alaya-P). With Thorax's rebellion, that formerly unbreakable status quo changed, and the Changelings accepted that change and tried something new, prospering in the process. It mainly boiled down to "change with the times, or fade away." In a way, you could say that the True Ancestors faced the same problem -- human support would have certainly kept them around longer, and maybe a solution to the Vampiric Impulse would have been found. here we find another difference between the two: While both groups face the same challenge (albeit in different forms), the Changelings rose above their original, now pointless purpose and changed with the times, thus securing their future and a new place in their world. The True Ancestors failed to change with the times, and thus faded to extinction, with no chance of recovery.

In a strange way, these differences between the two vampiric races further solidify and exemplify their connection. This is because of one main reason: their positions could have easily been reversed. It could have been the Changelings who died out, unable to rise above their original purpose and later uselessness; and the True Ancestors could easily have made the same relative decision to change with the times. In a way, this makes the tale of the True Ancestors, aliens and strangers on their own homeland, even more tragic; while it sheds whole new light on the Changelings, their hardships, and their ultimate reward for their troubles in the end.

In Conclusion
So, to summarize, both the Changelings and the True Ancestors were made by their respective Gaias to counter their respective Alayas. The True Ancestors ultimately failed and died out, and the Changelings, after being stripped of their purpose, languished for a time until they found a new place in the world and changed with the times. While similar to a degree, the one deciding factor - of whether or not to change with the times, outlined whether they would have survived or died out, and either race could have taken the other path and met the opposite fate to their canon one.
I will have to admit, this theory was a doozy! Even when I started writing it, I really didn't expect it to turn out the way it did! I really hope I did both the True Ancestors and the Changelings justice in this theory, but remember: this is all my opinion and open to interpretation. So make your comments below, tell me whether you liked it or not, and please try reading any TYPE-Pony Theories that I will post in the future!
Thank you for reading, keep trotting, fellow Bronies, and keep looking for that Root, fellow Type-Lunatics!

Ohhhh man, this is rad... I can't even think of anything to add here because I never even conceived of this. Super cool!

Another thing to add is that the Changeling decline hit much harder than the Ancestors. While at the very least individual Ancestors are leagues above normal humans, most Changelings probably have the power level of those Dead zombies, considering a whole platoon of them can be smashed by six girls without much combat experience. And one of them is a pacifist.

And no, capturing the Equestrian guards doesn't count. The Equestrian guard is a bunch of incompetent idiots.

To be fair to the guards, when peace lasts for a thousand years, you'll get lazy, and sloppy.

Mankind has had ten thousand years experience of warfare and if he must fight he has no excuse for not fighting well."
—T.E. Lawrence

When everyone, and everything everywhere is trying to kill you, you get good or get rekt. Ponies have it so made it's a wonder their will to evolve hasn't stunted.
This theory though, great. It's presented so simply that either fandom can grasp it😃. Now do the Crystal Heart. 😐

6372772
Not bad at all.

6374142
The Crystal Heart... hmm... I have an idea about that, but it should not be uttered here.

In the meantime, my next theory is in the works, covering the seemingly innocuous topic of Cutie Marks. Expect it out soon.

6372973
Regarding the Guards, I would say that we witnessed a secondary Worf Effect. We already know that consuming love ramps up a Changeling's power (look what happened with Chrysalis and Celestia!) How long had those changelings been there? Just because they are not Chrysalis, it doesn't mean that they can't benefit from love consumption the same way (in fact I would say the opposite holds true). How long had they been in Equestria as a whole? What's more, of all the threats that the Royal Guards were meant to expect on a daily basis, I don't think that a giant bug-pony invasion was on the list. Besides, the wedding was going great, as far as they knew, and it seemed that Shining's shield did its job! No need to worry! Let's celebrate our cap'n getting hitched, finally!

6374168 Shining Armour is what stallions want to be. The guy has a literal "happily ever after" in the works. Round of applause for living the dream!👏
I'm gonna stab at the dark and say cutie marks = origin&aliment🤔

OK, you got the Origin right, but it's a bit more complicated - that's all I am going to say for now. No need to spoil the theory now, right?