Derrick stared at the horse-like creature reflected back, then grinned at Champ. "I sure wouldn't have a chance to be female back home. I might as well go for species and sex change all at once." Champ chuckled. "You'll make a lot of stallions happy, not to mention me. I can't get you pregnant, so we'll just have fun. But it is likely one of the real horses will get you knocked up good. So you'll get to experience pregnancy and birth, as well as nursing your foal."
"In two weeks?" That seemed pretty odd to Derrick, who knew horses carried longer than humans, and what Champ was talking about might take two years! The centaur raised an eyebrow. "They promised you everything would be normal when it is time to go home, right?" Derrick shook his head. "I know what they really meant. Whatever I am will be 'normal.' But maybe I'll just entertain you. Letting stallions mount me is a choice." Champ nodded in agreement, though he also knew that a filly's instincts would have a lot of say in Derrick's decisions.
The centaur moved behind the horse-boy and started to massage the crack of his buttocks below his anus. "Changing your sex is easy, especially since you want it." Derrick watched in the mirror, watching his alteration to a horse proceed. "You could make me a mare without my permission?" Champ grinned. "Well, not without your permission, but all it takes is a little added desire and you won't be able to think of being anything else. As he said that, Champ slid his hand into a new opening and sent a weave of pleasure through Derrick's entire body. Derrick's eyes went wide as incredible lust filled him, not to drive his shrinking cock into a female, but to be mounted by a male.
Champ stood back and watched with satisfaction as the humanoid horse gained mass and leaned forward, dropping to four legs as he... no, she finished transforming into a horse. He waited, letting the change simplify and focus Derrick's mind m- by choosing to be a real mare, the former boy had also given up his human reason and intelligence. After a few minutes, the filly lost interest in the mirror and squatted to pee in front of Champ, her urine a marker and a signal that her mind was on mating.
Champ was ready to comply, already hard and hanging low. His equine cock drove into the mare, and he slid over her back as they mated. Derrick was lost in the frenzy of sex, the male lasting much longer than usual. She now had memories of mating with horses and centaurs, and the longer Champ was in her the more complete those memories became. He slid off the filly after several orgasms had left him empty and sated. The filly also looked content, and followed him out to a pasture where other horses roamed free.
Derrick's state of heat would be obvious to any of the males, some of whom were Resort guests enjoying what they thought was a temporary form. Some of them would mount Derrick and uncouple as normal animals, others might become the sperm that forms a new life within the mare's womb - only a few would return to humanity, just enough to spark interest in other guests.