"I wish you were a horse," she began, making Derek blush that remembered his favorite... "a mare, in heat no less!" she finished to his surprise.
"Mare?" he asked.
She shrugged. "What? I always wanted one and we can start a stables."
Before Derek could argue there was a sharp twinge in his his stomach that had him doubling over. He tried to stand again but he stumbled to hands and knees.
"Oh god, it's happening... Sonya I... I..." and as if he could not hold is anymore, Derek's pants ripped open and a swollen and already darkening with chestnut hair in a equine's powerful haunch. "Oh god, help!" he begged.
Sonya watched with a smile as her boyfriend's cock was pulled into a growing mare's cunt along with his balls and dignity. "And did I mention this will be permanent?" Sonya added with a wink as Derek's ears started to lengthen and curl in.
"Y-you can't! I'm a person!" Derek begs as he feels teats growing in and his face pushing out.
"Were, honey. You'll be the leading brood mare and don't worry, well taken care of. And fucked," Sonya giggled. "Bye bye, sweetie," she tittered as Derek's features lost themselves to the mare he was.