The stallion's ears pricked as he realized just how different this could all be. The satyr chuckled when he saw the reaction. "A mare? Come on, follow me and I will make sure you find out just how nice a stallion's cock can feel inside you." He waited until Derrick was trotting alongside, then reached out and started to rub the top of the former boy's erection. The long black shaft shrank quickly, not only withdrawing into the cave of Derrick's sheath, but drawing into his body to push back out under his... her anus. By the time they reached the stables, Derrick was a filly ready for her first breed. The former boy was trembling with need for a male, and her nostrils flared as a massive Shire stallion stepped out of his stall and snuffled in her direction.
The satyr patted Derrick's side and said "Go on. I promise you will be his perfect mate before he pushes into your cunt." Although the male seemed enormous even to Derrick as a horse, the satyr had not failed yet. She approached the male cautiously, then realized she was getting bigger. More than that - he... she twisted around and saw that her coat had become black like the stallion's. And she seemed larger, more... mature? Yes! Derrick felt the changes now, knew that she was a mature Shire mare instead of a filly, and even had flickers of 'memories' from prior matings, prior foals, being a horse for years.
The satyr watched the now fully-grown mare approach the stallion, a perfect match in years and coloring. If the former boy did not catch on, he would end up not only fucked several times by the male, but end up pregnant and overwhelmed by the mare's need and instincts. The two weeks would pass and Derrick would not want to be human again, choosing instead to spend the rest of her life as a brood mare for the resort. Which meant a bonus for the satyr for not only turning a human soul to that of an animal, but making a teenager into a mature and much-shorter-lived horse. And willingly, so the former human would be stuck in the animal kingdom forever more, no matter how many times he was reborn.
Derrick was oblivious to all of this, her new female parts slick and the need to be mounted almost blinding. She moved in front of the male and squatted, releasing urine that made the stallion's upper lip curl against his nostrils, then he bumped against her as he prepared to cover her. The satyr was watching eagerly, and Derrick felt grateful to the being for this wonderful opportunity, not knowing of his new 'friends' ulterior motives. Then all Derrick could think about was mating, for the stallion eared up and scored a direct hit, his swollen member driving deep into her cunt.
Did horses actually mate this long and this often? Derrick was in a daze after four long bouts of sex with the stallion. She felt different now, enjoying the sex, but aware of a basic change. The satyr had come over to rub her belly after each mating, causing a faint stirring inside her gut. But this time... there was no change. The satyr had nodded to himself and smiled as he patted Derrick's side. She knew then that she was pregnant. The knowledge might have scared Derrick the boy, but Derrick the mature mare simply knew and accepted this as a normal thing. She had birthed and nursed two foals... those memories were clear, unlike the fuzzy images that came to mind when she tried to remember being human.
The satyr led the complacent, contented mare to a stall, and spent a bit more time helping her pregnancy along. She was already more than a month pregnant, and over the next week the satyr would help the developing foal more, so that Derrick would give birth shortly before the two-week period was up. Even if the former boy still wanted to go back to being human, the maternal instincts for the new foal would be much harder to fight. The satyr was willing to bet that the first tug of the foal's lips on Derrick's teats would cement the former boy as a new and permanent mare.