Equis, Ponyville
"So, what exactly is this experiment?" Steve asked nervously.
Currently, Steve, Wanda, and Thor were gathered in the Palace of Friendship, having been called by Twilight just a few minutes previously. On Earth, there was no important mission to deal with at the moment, and Asgard was settled, so nobody's schedule had been interrupted. Currently, only Twilight, Starlight, and Spike were with them.
"It's simple," replied the alicorn. "There's a fairly complex 'species-change' spell that I've learned, and I was reminded of it when I reread 'Advanced Magical Transmutation.' What it does is magically scan a creature, then targets a second, temporarily molding them into the same species as the first. I've used it before, turning myself and the others into Breezies (long story), but I've never had the chance to try turning another species into a pony before. And since there's never been a recorded test on humans..."
Steve's jaw dropped. "You're not serious."
Thor chuckled, clapping him on the back. "Oh, come on, Captain! I've had this done to me in Equestria's past! What, are you afraid of losing your abs?" Steve glared at him.
"Um, Twilight?" asked Spike, tugging on her wing. "If Thor's really undergone this spell, why do you need to try it on humans? Aren't Asgardians and humans really similar?"
"From what Thor's told me," Twilight began, "Asgardians have similar biology, but they aren't exactly the same. For one thing, Asgardians are more attuned to magic, being exposed to it far more frequently in their daily lives. Probably explains why they live so much longer. We don't have a complete guide on how human bodies will react to the induction of magic, so this is important research."
"Wait, you don't know how it'll affect us?" asked Wanda, sweat beginning to form on her temples. "Starlight, you've thought this one through, right...?"
Starlight winced. "Don't worry. This isn't another one of my careless let's-use-magic-to-solve-everything' schemes," she replied, recalling her 'enchanting' of Twilight's friends with more than a hint of shame. "This is perfectly safe."
"There's a chance that the spell won't even affect your bodies," Twilight continued. "And even if it does, there's a leak purposefully built into the spell's matrix as a precaution. Once enough of the magic leaks out of your bodies, the spell won't have enough power to keep them in their current forms, and they'll just snap back to the way they were before. I've adjusted the spell so it won't last longer than an hour."
"Well, if you're sure," Steve replied, apprehensively tugging at his collar. He trusted her judgement, and if he was completely honest, curiosity ruled his mind. What would it be like, suddenly walking with all four limbs, and growing a new one (or more, depending on the type of pony)? Of course, the thought of becoming something else entirely...nobody would be completely comfortable with that.
"Great!" replied Twilight, oblivious to the soldier's anxiety. She turned to a spiral notebook, facing a page filled with equations she'd scribbled. "Let's see, just got to verify my math, made sure I accounted for your clothes...Yep! Everything checks out!" She put down the book, trotting over to the middle of the room, standing between Starlight and Spike. "Now, if you three could stand over there?"
She pointed to a wide open area, where the humans wouldn't be crowded by any furniture or other items. The three stepped into the designated area, Steve and Wanda swallowing deeply several times to try and hide what they were feeling. Thor needed no such relief, merely twirling Mjölnir on the handle loop.
"This isn't gonna hurt us, right?" Wanda asked, but her voice was so quiet it went unheard.
Taking a breath to prepare herself, Twilight ignited her horn, a faint white glow surrounding the appendage, in addition to her usual magenta aura. Spreading her wings to control her stance, Twilight launched a beam of pure magic at Steve, Wanda, and Thor, enveloping them in her magical nimbus. The three Avengers levitated a short height above the ground.
Turning to Starlight, Twilight cast the second portion of the spell, zapping the pink unicorn and extending the magenta light to her as well, though Starlight didn't float. Bracing herself, Twilight charged the final layer of the spell, aimed it at the three Avengers, and fired.
For a couple of seconds, nothing happened. Steve was just about to ask if something had gone wrong, when a faint tingle began to make its way across his bones. His eyes glowed bright with magic, and from what he could see, a similar phenomenon was affecting the others. Suddenly, a blinding flash of white light obscured the three from view.
The Equestrians were unable to witness the process, but the Avengers certainly felt it. Not that the species transformation was particularly painful, and it only took a few seconds, but the sensations were still there. Steve could actually feel his skeleton reshaping itself, as his spine bent forward, his skull extended to create a muzzle, and his fingers actually withdrew into the keratin covered piece that had once been his hand. A warm, muffling feeling smothered his body, as fur began to grow from his skin, and a faint popping impression above his rear signaled that a tail had grown.
With one more blinding flash, the process was over, and three new Equestrians dropped down onto the crystal floor.
Steve kept his eyes closed, grunting and shaking his head, trying to adjust his body to all the new sensations. Where his skin had formerly been bare, thin hairs emerged from every surface, lightly brushing against each other every time he moved. His brain registered the fact that he now had a fifth limb, though it didn't end in a hand or foot. He could even move his ears now, swaying them from side to side.
"So? What do you think?" Twilight prompted, and Steve heard a thump as some sort of object was set in front of him. He decided it was time to bite the bullet, and opened his eyes.
The object turned out to be a mirror, giving Steve his first look at his new body. Staring back at him was a large, Equestrian stallion, approximately the size of Big Mac or Shining Armor. His body hair was a brilliant sky-blue, a shade even lighter than that of Rainbow Dash. His mane was a version of his old haircut, with straight blond locks that didn't poke out from behind his neck. Standing sideways, Steve noticed that his tail bore fur of the same shade as his mane.
His clothes were a bit of a marvel. His Captain America uniform (minus the mask and shield, of course) had been adapted into a bodysuit fit for a pony, leaving little but his hooves and head uncovered. A small window was open on his flank, thankfully not exposing any parts Steve would've wanted to remain private. However, he could see the Cutie Mark that had appeared on his body, which was simply a picture of his trademark shield.
Attempting to get a better look at himself, Steve reared back, standing on his hind legs, but he wasn't used to the balance, and pitched forward, falling face-first into the crystal floor.
"Careful, now," Twilight said, helping Steve back onto his hooves, but doing a terrible job of keeping the chuckle out of her voice. "Wouldn't want you to hurt yourself."
"Hey, you try walking on two legs your whole life, and suddenly becoming a quadruped," Steve snarked. He carefully settled the appendages that were once his arms, feeling the cool crystal surface through the keratin layers of his hooves. He spread all four legs out, working out the proper stance for his new body.
"It's...bizarre," came Wanda's familiar voice to his left. Steve turned his head, discovering the sound now emanated from a pale red unicorn mare, standing a few inches taller than Twilight. Wanda's brown hair now cascaded down her neck and framed her muzzle. Her new tail was the same color and texture, fanning out in all directions, unlike the shorter hair on Steve's tail. Her limbs were a bit thinner than those of most ponies Steve had seen. Her Cutie Mark was a swirl of red light.
"Ah, I haven't had this form in so long," said Thor, in his booming voice. "It's just as I remember it."
Steve looked past Wanda. Thor had become a massive pegasus stallion, larger than Steve's stallion form. His coat was a steely-gray, and his blond hair had changed into an equally lush mane. His tail was in a pointed shape, like that of cartoonish electricity. His armor's new shape left his Cutie Mark exposed, which was, unsurprisingly, a thunder-cloud emitting several bolts of lightning. The largest difference, of course, were the two giant wings jutting out from either side of Thor's chest. Thor stretched out both of these, allowing the display of his maximum, impressive wingspan. Reaching out with a hoof, Thor called Mjölnir to him, catching the hammer through the loop on the handle. No longer requiring the power of the weapon to fly, Thor took off with a single flap of his wings.
And promptly pitched forward and crashed into the crystal wall.
"Oh my goodness!" exclaimed Starlight, rushing over to the wall and telekinetically lifting Thor off, placing him back on the floor. "Prince Thor! Are you alright?"
Thor wobbled a bit on his hooves, but seemed otherwise unhurt.
"Should've known I'd be out of practice," he chuckled. "I'll be fine. And there's no need for titles, Starlight." He looked around the room. "I don't suppose I could get some additional flying space?"
"Of course," Twilight answered, using her magic to open the doors.
Thor proceeded out of the castle with little trouble, but Steve and Wanda quickly realized they'd have to relearn how to walk. In their new bodies, with twice the number of appendages touching the ground, they found themselves stumbling and tripping, unsure of how and when to set each leg down. Twilight and Starlight rushed over to them.
"Don't worry," said Twilight. "It took me time to get used to a human body. You'll learn."
"Easier said that done," Wanda quipped, as she attempted to take another step, but failed to move her hind leg in addition to her foreleg, resulting in absolutely no forward movement.
Eventually, Steve and Wanda managed to get enough coordination to actually walk out the front doors of the castle. On the front lawn, Thor was busy becoming reacquainted with his wings, flapping them in a steady beat to keep himself floating a few meters off the ground.
"It's definitely coming back to me," the Asgardian declared. "I'd never outrace Miss Dash, but I think I could manage the weather again."
"I'm still getting over the fact that Equestria actually manufactures their own weather," said Wanda, wobbling across the grass.
"I know, it's the perfect place for someone of my talents," Thor chuckled. "Watch this."
With a flap of his wings, he took off towards a nearby cloud, which he promptly grabbed with both front hooves. Holding the cloud in place, he bounced on it with both hind legs, eliciting a steady stream of water droplets from it. Thor spread his wings once more, pushing him forward, dragging the raining cloud along with him, ensuring that the front lawn of Twilight's castle got a good watering.
"Huh," said Steve. "I don't think I was jealous over the fact that I can't fly until now."
"I can't say I blame you," said Twilight. "Before I got my wings, I found myself envious of Rainbow on occasion. Still, earth ponies are powerful in their own right. Applejack especially taught me that."
"What are my new abilities?" asked Steve, curious.
"Earth ponies have the ability to feel conditions in the soil. That's why so many go into farming or geologic jobs. Additionally, they're the strongest breed of pony, and since you were already enhanced, you're probably the strongest an earth pony could ever become."
Steve felt his muscles. They certainly did seem thicker and harder than they were in his human body, and he was definitely stronger than he had been.
"Why don't we test it out?" said Spike. He pointed to a small tree growing near Twilight's castle. "Try bucking that. See how many branches you knock loose."
Steve followed, walking up to the tree, and facing away from it. Mimicking the motions he'd seen Applejack perform previously, he raised both hind legs, and slammed his hooves into the tree with full force.
Upon impact, a crack like that of breaking bones come from behind him, and Steve whirled around to see the damage. While he hadn't managed to snap the tree, he'd dented it very deeply, leaving a massive, hoof-shaped pit in the middle of the trunk. Steve blinked in amazement, unaccustomed to being quite that strong.
"Wow," said Twilight, just as surprised at the sight. "Stronger than the average earth pony, at least."
She levitated a notebook and pencil out of nowhere, beginning to scribble notes down. "This requires further study!" she said eagerly.
Spike groaned. "Here we go again."
Starlight and Wanda watched Twilight's antics unfold with amusement.
"So," Starlight tentatively said, indicating Wanda's new horn, "do you have any interest in learning to use your new abilities? I mean, magic takes a long time to learn, and since this is the first time you've ever been able to access Equestrian magic, and—"
"Starlight, you're going into overdrive again," said Wanda, familiar with some of the unicorn's more nervous ticks by now.
"Right, sorry," said Starlight. "Maybe we don't have time to teach you magic during this visit."
She stared at Wanda's horn for a short time, before another idea occurred to her.
"What if you tried to focus your own powers using your horn as a conduit? They seem to operate in similar fashion to magic. Maybe we could use that as a starting point."
"Well..." Wanda contemplated turning her down, but found that she had no real reason to. After all, she wasn't staying in her pony form forever, but if the Avengers kept spending time on Equestria, she might have to return to it for a mission. Additionally, Twilight only wanted to study Equestrian magic and its effects on humans. What was the harm?
"Okay, I guess. What do you do in order to channel magic through your horn?"
Starlight was quiet for a minute, contemplating her answer.
"What you do is...you know what? This is one of those things that is so simple in your head, but when you try to explain it to somebody who has no frame of reference for it, your brain is just at loss for words."
Wanda poked her horn a few times, trying to feel the sensations of the new appendage. She couldn't move it, but could focus her sense of touch in it to feel the surroundings. Glancing at a loose rock lying on the ground, Wanda held up a hoof to silence the still rambling Starlight.
"Forget it, I think I've got an idea."
She reached into her own mind, opening the same doors she always did when she was activating her powers in her human body. She felt the familiar energy flow down through the center of her being, going for its traditional pathway down her forelimbs. This time, however, Wanda put a leash of the loose power, directing it instead through her brain, down into her horn. As she did so, the familiar red wisp faintly materialized around her horn, like a normal unicorn's magic glow. Starlight stared in surprise.
Wanda continued through the motions of her powers. She imagined her hands stretching out, seizing the rock and lifting it into the air. She curled her neck, pointing her horn directly at the rock as she did so, mimicking the hand gestures she did in her normal body, which helped her direct objects exactly where she wanted them to go. Tendrils of red light reached out, wrapping themselves around the rock. Wanda leaned back, smiling with pride as the rock followed her movements, rising into the air.
With a sudden grunt of pain, Wanda slumped back onto her rear, front hooves rushing up to clutch her skull, as the red energy dissipated, dropping the stone. Starlight scurried over to her friend.
"You alright?" asked the unicorn.
"Yeah, just a small headache," Wanda insisted. "Happened the first time I used my powers, too. I think they're confused at being forced through a new appendage, or all the magic in this new body is throwing them off."
"No matter. We can work on that," assured Starlight.
"So, one question," said Steve, trotting over with Twilight by his side. "You used Starlight as the base creature for the spell, but only Wanda turned into a unicorn. Shouldn't we all have been unicorns?"
Twilight opened her mouth to answer, but Thor cut her off, as he floated down to the group. "I believe it's based on the mentality of the subject," he said. "Whenever Celestia used the spell on us, I always became a pegasus, and Loki became a unicorn, no matter who the base being was. Loki was always a skilled mage, and my powers are rooted in weather manipulation. Lady Maximoff's abilities are similar to those of a unicorn, so that explains her transformation." He looked at Steve for a while. "I'm not sure about Captain Rogers, though."
"Maybe he just knew he didn't need wings or a horn to kick butt!" cheered Spike.
One hour later, the effects of the spell had worn off, and Steve, Wanda, and Thor were transformed back into their normal selves in another blinding flash of light. The group gathered by the mirror portal, ready to return to Wakanda, as Twilight scribbled more notes on the experiment into one of her many notebooks.
"Well," she announced. "This has been enlightening, thanks for your help."
"My pleasure," said Steve.
"It felt pleasant to be a pegasus again," Thor agreed.
"It was...interesting," Wanda admitted. "Do you have any plans to try the spell on the others?"
"Eventually, if they'll let me," Twilight replied.
Steve's brain was suddenly overcome with the mental image of Tony as an Equestrian. Maybe that's why he fell on his back, shaking with laughter.