Sherry shivered a little as the heat grew in her pussy. She wanted to be mounted by this stallion more than anything, to be a horse like him and bear his foals. She was holding on to her human mind, but she still felt the growing desires and instincts as her transformation sped up a little, red fur slowly covering her body as her face pushed out into a muzzle.
Ferdinand smelled the scent of the mare and backed up to let her move into position. Sherry leaned on the fountain and pushed her tail off to the side, revealing her now fully transformed pussy and his cock became fully erect.
"Ohhh... mount me... make me a mare..." Sherry said with a moan as she wiggled her flanks at Ferdinand. "I want to be a mare more than anything..."
Ferdinand needed no further prompting as Sherry felt the weight of his body pressing against her and the feeling of his cock pressing against her entrance. She moaned as Ferdinand thrust into her as her body began to barrel out into a horse body as her arms and legs formed into horse legs.
She moaned as she felt the stallion thrust deep into her, his horse cock felt strange and alien, but she didn't care. She moaned louder as her hair grew along her neck and formed her new mane as Ferdinand thrust faster and deeper as her moans turned into whinnies and neighs as she started to move her hips with the thrusts.
She gasped as she felt his large horse balls slapping against her as the last of his massive cock thrust into her. She whinnied loudly as her muzzle finished its transformation, her body responding to it as she felt her pussy start to clamp down on his cock. She gasped as she felt the stallion's balls stiffen and after a moment he cummed hard inside her, her vaginal muscles drawing out as much of it as they could as she neighed loudly.
As Ferdinand dismounted her, he nuzzled her gently. She looked at her reflection for a long moment, staring back was a beautiful red coated mare. She could feel the combined juices dripping down her hind legs as she neighed happily, this was perfect, she was her favorite animal and she loved it.
"Well, you're looking great," the attendant from before said as she approached the two and patted Sherry on the nose. "Do you like your new body?"
Sherry tried to reply but only a neigh came out so she nodded. She smiled and put a bridle on her and started to lead the two away and back towards the stable.