"Amazing it is!" Hephaustus started moving around Derrick, using his strong hands to rub and stroke the thick equine body below Derrick's waist. "You will be Shire-based, the biggest, most powerful centaur here!"
The powerful massage felt really good, and started a strange rippling sensation that spread over and through Derrick's body. Muscles thickened, and he started to get taller as his equine legs lengthened. The horse hide under Hephaustus's hands began to darken to midnight black, the coat getting silkier and thicker. When the blacksmith started on Derrick's genitals, the former boy gasped and had to brace himself as he was worked up to a rapid climax. Derrick threw back his head and gave a kind of strangled whiney as he erupted, massive Shire testicles emptying with several powerful jest of semen that splattered on his forelegs and chest.
He was still panting from the sexual release when Hephaustus stepped back and admired him. "You look incredible. One more suggestion. Let me make you all black on top so you match your horse parts."
Derrick thought about that a moment, then nodded. "OK! That sounds pretty wicked!" He was expecting to have his human parts become African, but as Hephaustus ran his hands over Derrick's massively muscled chest, his skin turned true black. When Hephaustus finished, he gestured for the centaur to follow him over to a large, still pool. Derrick looked at his reflection and grinned, large flat white teeth brilliant in his coal-black mouth. His hair, and skin matched his horsehide, and looking closer, he realized he now had the liquid-brown eyes of a horse, huge and wide-set under thicker brows. There was nothing recognizable from his old human self - he was now a powerful, mature centaur, probably the largest of his kind anywhere.
"This really is amazing!" His voice was deep and gruff, with a little horse grunt/whicker sound behind it. Hephaustus nodded. "My best work ever. You should have some fun with this new form before you go back to being human." The blacksmith gave him a conspiratorial wink. "That is, if you want to ever be a puny human again. There is always a place on staff for centaurs. And if you want to explore your new sexuality, several of the mares here will be happy to introduce you to stallionhood."