Hephaustus chuckled. "Whatever I want? You are a brave one."
Derrick shrugged. "Anything you do is only for as long as I am here, right? When I go home I'll be normal again."
"Well, yes." Hephaustus grinned and patted Derrick's rump. "But normal may not mean the same thing when I am done. I see the stallion's interest in you, and I am thinking you want to experience something very different. So I would make you into a mare, the most amazing Shire mare this island has ever seen. Are you willing to let me do that?"
Looking back at the huge stallion snuffling at his side, Derrick looked thoughtful. He hadn't thought about experiencing sex as the female, but it was certainly something he couldn't do away from this place. And the idea made him feel... interested. "A mare? What happens if he gets me pregant?"
Hephaustus chuckled, his hand dropping to massage the firm jutting rump muscles below Derrick's tail. That felt really good, and oddly loose and damp. "Not 'if,' my young friend. You will have a beautiful foal, which you will nurse until it is weaned. And if you are unable to resist the charms of our stallions, you will be pregnant again before you have finished weaning your foal. I can make you a very long-lived brood mare, even tweaking reality so that you will have dozens of foals already birthed and a hundred years of more ahead of you. And when you finally break the cycle, you will start all over again as boy or foal, or both."
"Tweak reality?" Derrick shivered as Hephaustus' fingers slipped into the crack between his buttocks, below his equine anus, and started rubbing a very sensitive, wet spot that got larger. The stallion's ears perked, and he shifted back to sniff and lick at the same spot. "Oh, man!" A powerful ripple of pleasure made him gasp. "If I had dozens of foals already, wouldn't that make me have been a horse longer than I have been alive?"
"The reality would be that of you born as a mare even before your own human great-grandfather was an infant." Hephaustus grinned. "A reality that will allow you to experience this amazing life as a mare and still choose to resume your human life when your family comes here."
Derrick felt confused and disoriented, and staggered slightly as the ground seemed to shift under his hooves. Then he blinked in astonishment. All of the spa buildings were gone! In their place were rolling fields, a crude-looking ancient farm, and horses. Event he air smelled different. He gasped "Where are we?"
"The same place, just two hundred years in the past." Hephaustus's fingers were deep inside Derrick, stimulating what had to be a very sensitive mare's cunt. The stallion whinnied, and Hephaustus withdrew his hand just before Derrick squatted and urinated out of the new opening. Surprised, Derrick started to apologize, only to have the words catch in a changing throat. His human parts were transforming rapidly, arms shrining away as his face pushed out into a muzzle. Hephaustus stepped away as the stallion licked and nuzzled the black lips which Derrick could feel clenching and releasing. Then the huge male pushed forward suddenly, driving his fully-erect shaft against Derrick's rump. It caught once against his anus, but slipped down and found the mark on the next try.
Derrick gave a shrill whinny as the stallion drove deep and began to thrust, the last traces of centaur fading away into a magnificent Shire mare. Pleasure filled Derrick's mind, instincts taking over. Mating was quick and brutal, but Derrick was thrilled by the experience. When they finally stood, sweating and spent next to each other, Derrick looked around and saw some peasant boys working a field, and a man guiding a plow behind a huge horse. The island 200 years ago. She realized that Hephaustus had created the reality without waiting for her answer, but find that she did not mind. It felt normal being a horse - a now-pregnant mare. In fact, this was familiar to her - many foals had come from her womb, and she knew many others would before she finally resumed her life as a human boy two hundred years from now.