Mary stood in front of the full length mirror, and watched as small bristle like hairs had started poking through her skin. She ran a hand down her sides and back, and felt more hairs growing. Soon her entire body was covered in a thin layer of donkey fur. It got longer and thicker closer to her crotch, and a happy trail was formed. Her ears felt funny, and soon they stretched out. They moved upwards to the top of her head, where they soon became furry as well. Mary tugged on them, and rubbed the fur. It felt good. Really good.
Then a sharp pain in her rear drew her attention. She gingerly felt above her ass, until her fingers touched a hard bump. The bulge soon pushed further out, until there was a long hairless tail hanging from her. It didn't stay hairless for long, as the fur that was slowly thickening on her body spread down her new appendage. Longer hairs grew on the end, until she had a regular donkey tail.
It took some doing, but Mary was eventually able to flick her tail back and forth. She giggled, and felt her body heat up for new reasons. She was getting really turned on by her transformation. Her sex dribbled girl juice onto the cold floor. Mary started to reach her hand down to touch herself, but found her hands were feeling awfully stiff.
She watched in amazement as her fingers fused together, and changed shape. They became much thicker, and her nails spread out. Black keratin formed as her fingers fattened up, until they looked like someone stuck sausages on her hands. Once the stiff feeling subsided, Mary was left with two fingers, and one thumb, all ending in blunt hoof-nails.
The same thing happened to her feet. Her toes stretched out, and the nails overtook most of the appendages. Her legs cracked, and switched to a digitigrade stance. Mary leaned on the bathroom vanity for support, until she was able to stand on her new hooves. It was difficult, as her body had slowly been gaining weight as her transformation progressed. Her lower half expanded, while her upper body gained muscle. Not a lot, but her arms and legs were much more muscular than before. Which was good, given that she had more to carry around.
But the most unexpected change came when her chest tingled, and slowly began to recede. After a few seconds, Mary was left completely flat chested, even without nipples. That freaked her out to say the least. On shaky legs, she went back into the bedroom to check the vial.
"Oh crap." she whispered, rereading the label. "Jackass", it had only just dawned on her that a jackass was a male donkey, not a female.