You felt a sensation between your hind legs as your body started to change again. You could feel your equine genitals shifting as your vagina closed up and your organs formed your new balls and shaft that slid into your new sheath. Your girlfriend didn't even notice as your body changed to that of a handsome stallion.
You could smell the changing scent of the two humans that were with you and Sofie. You looked at the grey mare and grinned as you saw Carl's body transforming into that of a black-furred mare, the two humans unaware of their transformations. Sofie swished her tail, giving you a view of the winking folds of her vagina behind her tail.
You whinnied lightly at the scent, knowing that you'd soon have three mares to mate with. Your equine instincts were starting to overpower your human thoughts as you imagined mounting and breeding the soon to be horse mares.
Eventually, the four of you stopped in a clearing next to a creek as they stopped to let you and Sofie graze and drink while they enjoyed the day. You pretended to graze as you eyed the two who were watching and laughing as their clothes finally tore without them even noticing or caring.
Finally, their voices turned into whinnies and neighs as their transformations completed. They looked startled for a moment as they looked around confused for a long moment, trying to figure out what had happened. You grinned and trotted over to them, their tails swishing back and forth to reveal their new equine equipment, two beautiful mares, one black and one white with a yellow mane stood before you as your shaft slid out of its sheath.
You were ready to take your first mares as a stallion. Your horse mind was pushing you towards it as the mares raised their tails in anticipation, the former humans seemed to not even remember who they once were.