Tachyon's Terrible Tincture

The Trainer stood alone in the park. Tachyon was once again late, and he was once again being stared at by other Umamusume, all of whom seemed to be whispering to eachother that the “creeper” was back at the park.
Letting out a long groan, he finally decided to leave. Tachyon had decided to skip out on her training, again.
“Sometimes, I wonder who the trainer in this relationship is.” The thin, wiry young man muttered to himself, as he made his way out of the park.
“Hold it!” A familiar voice called out to the trainer.
The trainer’s head swiftly turned, as he locked eyes with…
“Tachyon! Finally, I was wondering when you’d-” Before he could get another word in edgeways, the horse-girl shushed him.
“Today’s training is being called off! For I’ve made a BREAKTHROUGH, one that will CHANGE THE WORLD! And you- Yes you, my loyal, lovely guinee-trainer, will be the first to experience, and also test this MARVELOUS CONCOCTION!” Tachyon let out a mad sounding laugh, as she produced a bubbling vial of pink liquid from within her lab coat, “Now, I’m not actually sure if this’ll work, however I’ll make sure to document every single oddity that occurs once you down this lovely mixture!”
“T-Tachyon, I… Ergh, fine. But we’re getting in some training while you monitor me for whatever this thing is.” The Trainer snatched the vial out of the slightly smaller horse girl’s hands, popping the cork with his thumb and taking a long swig, “Gergh! That tasted rancid!”
“Fascinating, fascinating… I seem to have forgotten to add any flavouring to the mixture. Noted.” Tachyon was lost in her thoughts, as she began to scribble something down onto a clipboard she had pulled from her satchel.
“...Tachyon? What’s uh- What does this thing actually do, anyway? Another serum to make my hair change colour? Will it make my legs glow…? Or is it going to make me run faster or something?” The Trainer’s voice slowly began to fill with a slight amount of worry. He was used to being drugged by Tachyon after almost a year of training her. At this point, he knew that simply accepting whatever she wanted to test on him was the quickest way to get her to focus on her training. He knew she’d never do anything to actively hurt him. At least, not on purpose. However, he was getting ever so slightly worried about what exactly this was supposed to-
“Oh, I’m so glad you asked! See, this’ll alter ANYONE’S internal biology to that of a Umamusume! You SHOULD maintain your current physical body as well! Can you IMAGINE what we could do with a serum like this?! The whole WORLD could be in the palm of our hands! Ordinary, twiggy little humans like you would be able to run as fast as a car, and have the strength of a bodybuilder!” Tachyon gleefully began scribbling down more notes onto her clipboard, an absolutely manic smile spreading across her face.
“T-That… Wow. That’s… Incredible. But you’re SURE this isn’t going to do anything… C-Crazy to me, right Tachyon? I mean, none of your… Experiments have had any long lasting effects on me so far, so I sort of trust you, in a weird way.” The trainer’s voice filled with uncertainty, “However… Having the abilities of a Umamusume as a human? I’d be able to do so much more with you! Train you even better! Even if I’d lack the raw speed you’ve got.” The trainer seemed to briefly lose himself in thought, as Tachyon quickly poked her head up from her clipboard.
“You understand, you understand! This is why I chose YOU back on that track! There’s no need to worry, dear guinea-trainer. You won’t go growing horse ears or anything. I’m ninety nine percent sure this will have no negative physiological effects on you. You’ll appear as boney and physically underwhelming as always, while hardening internally!” Tachyon beamed up at her trainer, who let out a sigh of relief.
“Well, that’s a relief. Anyway, let’s get to training. Seriously, we can’t keep on slacking. You can monitor this while you’re running laps.” As the trainer spoke, his ears seemed to slowly begin sliding up the side of his head, before thickening, and twisting outwards. Soon, they were covered in fur, and…
“Oh. Well, I did leave a one percent margin of error!” Tachyon began scribbling notes, “Don’t worry, trainer! I’ll make sure to document every single grizzly detail!”
“...What are you talking abou-” Before the trainer could finish his sentence, his eyes widened, as a RIPPING nose could be heard. A brown horse tail had burst through his trousers, and was now gently swaying behind him.
“T-TAAAACHYON! Oh my GOD! Please tell me you have an antidote for this?! I look- I look like a Umamusume! A MALE Umamusume?! Is that even possible?!” Genuine panic filled the trainer’s voice and eyes. As he panicked, his short, messy hair began to rapidly thicken and lengthen. His face too, began to grow softer and more feminine.
“...I’d say er, no. No it’s not possible. At least the male part. Fascinating. It seems as if your body is correcting itself to fit the new abilities my serum presumably allowed you to develop. I wonder why such a thing would occur? A metamorphosis of the shell, to match what I’ve changed internally…” Tachyon tapped her pencil against her chin, consumed by a morbid curiosity as she closely observed her trainer.
“W-What the hell are you talking about- EEP!” The trainer’s voice suddenly raised a few octaves, becoming distinctly feminine, before his height was suddenly slashed. He now stood at roughly the same height as Tachyon.
“T-T-Tachyon- Oh, GOD! My- My… MY-” The trainer fumbled with their now loose pants, as a look of absolute dread crossed their face. All the while, two budding mounds formed on their chest.
“Remarkable! Well, I definitely didn’t set out to create a sex changing drug, however I suppose when administered to male humans, the serum corrects their biology to match their altered cells! By god, this is GROUNDBREAKING!” Tachyon poked the trainer’s breasts, which seemed to have settled at a slightly large C-cup.
The trainer let out an “EEP!” of horror, as he- No, she stumbled backwards, furiously blushing as she did so.
Their hair was now a thick, messy brown, bangs covering their bright blue eyes. Their face had softened and morphed into something mousy, and soft. Meanwhile, her thighs and hips had expanded quite a fair bit, giving her a modestly curvy look underneath her now baggy, ill fitting clothes.
“T-T-TACHYON! Y-You need- Y-Y-You need to change me back-bwuh- GUH!” As the trainer rapidly stumbled back, she proceeded to trip over her own shoes, now a few sizes too big for her. She fell back, slamming into the ground with a thud.
“...Oh. He- She? Knocked herself out… I should probably take them to the nurse.” Tachyon hauled their trainer over their shoulder with ease, and began to carry the horse girl towards the nurses office.
----
“...So there’s nothing you can do?” The former trainer asked with watery eyes.
“Nnnnnope! SORRY!” Tachyon spun around in an office chair, suddenly planting her feet on the ground to stop herself from spinning further, “Look on the bright side! They offered you a place here in the school until they figure out a way to fix this, as a STUDENT! Now you get to RACE with me! You can even STUDY with me. Now I get access to my faaaaavourite guinee-pig twenty four seeeven~!” Tachyon began to slowly creep towards the former Trainer, who found their thick thighs quaking as Tachyon grew closer.
“I-I don’t consent to anymore of y-your experiments, not unless they’re related to turning me BACK TO NORMAL!” The former trainer managed to sputter out.
Just then, the door to the infirmary opened.
“Oh, hey! Great news, there’s already a trainer who’s scouted the BOTH of you out!” A Umamusume happily exclaimed, as the former Trainer suddenly frowned deeply.
“Oh, I’m sure little miss “Trainer” here’d love a trainer of her own, isn’t that right?~” Tachyon teased, as she once again prodded at her former trainer’s new breasts.
“TAAAACHYON!” The former trainer shrieked.