She's a Centaur

Kendra was lucky enough to make it all the way to 3rd period by the time she started noticing anything wrong

Kendra sat in her class fervently itching her legs. She couldn't figure out why they itched and she was debating calling the doctors again, even though she knew they were watching her every move.

She tried to focus on her class as her legs itched more and more. It became harder to focus as she tried to determine what had caused this. Then it hit her. My formula! Her mind shrieked. She had forgotten it and now any moment she was going to transform into her "true form" as the doctors had called it.

It had started on her eighteenth birthday, she had felt the same itch, the same building heat. Her body had spontaneously and rapidly changed into that of a centaur, once upon a time a majestic creature, but now? Now it was an inconvenience. Kendra had been stuck in that form, until they met the doctors at the lab, who had given her parents the formula. Once a day, ever since, Kendra had to drink the formula, or else she would change back into that beast.

It would have been horrible if she turned into that thing in the middle of class. She could already feel the fur growing and her breasts slowly gaining size. Her shoes straining as her feet shifted.

The change would either be quick and painful, or long and uncomfortable, she never knew until it was done. She had to get out of class.