As Nia finished changing, Mike heard a gasp from the side of the barn. Looking over, he could see a mop of hair and a pair of eyes, peering out from behind some hay.
Mike heard a thump, and looked to see Nia fall to the ground, fainting from the shock of her transformation.
He turned back, and saw that the figure had arisen. It was a young man, probably in his late teens, and he bore a strong resemblance to Nia (well, to her human half, anyway.) The young man had a hungry and determined look on his face. He approached, announcing, "I am Arro, and Nia is my sister."
"I... I had no idea she would transform..." Mike stammered.
Arro approached, and walked around Mike, appraising the centaur's powerful body. Arro leaned in, and whispered words to the saddle that Mike could not hear.
"Centaur!" Arro said, "Flex your biceps for me!"
Mike had no choice but to comply, and raised his arms up and flexed, showing off his impressively large biceps.
"Excellent," Arro grinned. "Now tie up my sister very tight, and gag her!"
Mike was shocked by the order, but immediately complied, tying Nia up with very strong ropes, and gagging her thoroughly.
"Now," Arro announced, an edge of giddiness in his voice, "I want to be as big and powerful as you! So I want you to fuck my ass for the next hour!"
As tired as he was, Mike immediately became hard, and strolled over to the little human. Arro pulled off his clothing, put them in a neat pile, and knelt, exposing his ass to Mike. The centaur, for his part, obediently thrust his over-sized horsecock into Arro's tight ass, eliciting a shriek from the lad. Mike hesitated, but Arro ordered him to get going. And go Mike did. Although he was tired from ploughing the field, and then Nia, he ploughed Arro like his life depended on it. Arro whispered encouragements like, "Yes, yes, that's it," and, "Make me a big fucking centaur!" At some point, Nia awoke, but the bonds and her gag meant she could only thrash and whimper, her powerful centaur body unable to burst the bonds that Mike had so powerfully tied.
Finally, when the hour was up, Mike withdrew, utterly exhausted. Arro shakily rose from the ground, his wobbly legs barely able to hold him up. A wave of pain and dizziness hit him, and he collapsed back to the floor. Slowly, his body began to inflate, as his lower half became a powerful stallion, covered with silky brown hair, while his upper half developed into that of a powerful human. Arro grinned madly, as he rose from the ground, feeling his new body.