Katie, of course, had no choice but to agree. The Shopkeeper handed her a cup of tea. He told her to drink it, and then return home.
"But what if someone sees..."
"No one will see you. The changes will disappear slowly, by morning you should be back to your normal self. So just drink your tea and go back wherever you came from. But remember, you and I have a 'date' come the next full moon." The Shopkeeper winked before ushering Katie out the door. She barely had time to swallow before the Shopkeeper pushed her out the front door.
She blinked, and suddenly was standing at her own front door. More confused than ever, she whirled around but the man and his shop were gone. She was standing at her house, naked and in her horse form, wondering what kind of magic the Shopkeeper used to land her home.
"Maybe this is all a bad dream. I'll wake up and everything will have been a terrible dream." Katie hoped. But knew that you don't declare something is a dream in the midst of one. The front door was open, and Katie clopped inside. She was so tired, and in no mood to try and walk upstairs. Not with hooves. Walking down the stairs early had proved to be enough of a challenge.
Katie wandered into her living room and collapsed on the couch. She remained there until morning. Her mind a million miles away, while her body started to feel the effects of the vial the Shopkeeper had slipped into the tea while Kate was in the bathroom. In tandem with her changes, he too reverted to his human form.
But his natural human body wasn't the old man Katie had encountered earlier. No, that had been the effect of another vial he had taken. The animal characteristics melted away, and revealed a handsome young man who, much like Katie, hadn't heeded the previous Shopkeeper's warnings. He too was cursed to transform into a stallion form, and could only change back if he received a mare's juices.
Katie moaned softly in her sleep as her body heated up. The first thing to disappear was the "fur" all over Katie's body. It was pulled back, as if someone was tugging an invisible string that made all the hair retreat back into her skin. Once her fur was gone, Katie's mane shrank back into her skull. The long hair on her neck retreated, but more hair grew on the top of her head. The heat then centered on Katie's chest, and she subconsciously wrapped her arms around her chest as if to shield it from the pain. But that didn't do any good. Her breasts and nipples began to deflate back to their original size. Had she been awake, Katie would have heard a loud CRACK, as her hooves split into five distinct sections. The hard nail shrunk and shrunk, eventually only covering the end of her returning toes.
A sharp pang of pain hit her tail, which started lashing about in Katie's sleep. It continued to whip back and forth, even as it was drawn back into her body. The long hairs fell out, leaving a fleshly appendage about five inches long. But even that was sucked back into Katie's body, until nothing but flat skin was left above Katie's ass, which was also in the process of shrinking.
A rush of pleasure hit Katie's unconcious body as her pussy and asshole started to reform. The large, puffy mare slit was sucked back into Katie's body, as was her anus. Both returned to their light pink color as they became smaller and smaller. The teardrop shape disappeared, as Katie's labia pulled back to their normal shape. The only thing different was the amount of pubs Katie had now. Light brown hair covered her entire pussy, and even went up around her human asshole. A side effect of being a werehorse, extra hair.
The most painful changes came toward the end. The worst of which was when Katie's muzzle began to recede. The process of her skull reshaping itself was slow, and in addition her larger, equine teeth fell out to make room for her smaller, sharper human teeth. After hitting the floor, Katie's old equine teeth turned to dust, leaving no trace behind. Once her face finished reshaping, the painful changes continued to her legs. The digitigrade stance cracked, and muscles shrank away to nothing.
Finally, the bones Katie's legs, and the rest of her body, stopped contorting and shrinking. She was finally back in her human body. Though she was a tad hairier than she'd been before (and not just around her crotch). Her arms and legs had very obvious peach fuzz on them, though under the cover of night no one would notice.
Katie sighed as the transformation ended, her body and mind both readjusting. She was dreaming pleasant thoughts, thoughts about being human. But the dream would take a sharp turn and cause her to bolt upright in the morning, covered in sweat and breathing heavily.
The dream came to a terrifying end when Katie looked into a mirror and saw the face of a horse staring back.