The change centered around Ty's dick. He felt a tightening sensation and rushed to get his pants off when he realized what it was. He got his pants off but there was no time for his underwear. The waistband snapped and the tatters of his underpant lay scattered at his feet.
His dick was already a foot and a half long and growing. It had also gotten hairier, his balls were the size of baseballs and growing like they were waterballoons being filled.
The feelings were really turning him on and he started to rub his dick, it was like running his hands up and down a fleshy baseball bat. His balls tingled and injected the semen into his dick, it seemed to take forever for it to travel up through his dick and erupt fromt the end.
His pleasure was cut short by the pain in his ass, a tail was forcing itself out of his back. He could feel his spine lengthen and arched his back as it grew out. the fur finally began to grow, straing on his ass and spreading below his waist.
His shoes were history as his feet changed into hooves and ripped them apart. Two new legs burst out of his midsection, he fell onto his new legs and the change finished. Within a few moments the rest of his body had changed to reflect whan was now below his waist. His chest grew strong and a resoectible growth of stubble burst out of his face.
Ty was a fully grown, magnificent centaur.