"What's the worst that could happen?" Katie thought popping the top off the vial and downing what remained in one gulp.
Katie tossed the empty vial to the side and waited for her changes to take hold. The tingling sensation slowly built until it was all over her body. Her skin prickled, and she saw hair creeping down her legs. Her already equine genitals became coated in palomino hair. And then her cock slide from its sheath once more. Katie resisted the urge to stroke the shaft, and watched as hair, now fur, ran all the way down to her transforming feet.
Her toes merged together, as all but one nail fell off. The remaining nail grew until it covered the fleshy stub that was her "foot". She could feel her bones rearranging themselves. Her foot hardened just as her legs finished expanding. They were gaining mass, but in the form of muscle. A sharp crack echoed off the tiles, and Katie's knees became inverted. She was forced to sit down on the edge of her bathtub as her legs reshaped.
A tight pinch above her ass made her jump, and then stumble as she wasn't used to her new hooves yet. Katie caught herself on the sink just before she crashed to the ground. She'd pinned her tail against her large ass. As Katie rubbed the sore appendage she noticed her hands were different. Her fingers felt stiff, and she couldn't move them as easily. Holding one hand up to her face, Katie saw that they too were transforming. Her nails were much thicker. Blackness panned out from the base of her nails, until it was like she had miniature hooves on each individual finger. Katie took shaky steps towards the mirror to see the rest of her changes.
Fur had grown over every inch of Katie's body. A thin white mane rested on her neck, and her ears appeared to be a bit longer. Her jaw looked harder, more angular. Her nose was a bit thicker, and her nostrils larger. Bristle like hairs stuck out from her chin and nose. Katie also felt her tongue engorge inside her mouth. It still fit, oddly enough.
But the most unexpected change came when her chest started to deflate. Her beautiful breasts shrunk and shrunk, until she had well toned pecs instead. Her nipples were big and black, at least for a guy they were big. Compared to her female body, they were small.
But Katie was forced to accept that she couldn't call herself a "she" anymore. Once the transformation was finished, she stood in front of the mirror a half horse, half man. Aside from the equine features, there was no denying she was male. Nothing remained of her female self, except for the long hair that was more like a mane.