Derrick woke with his partners, stretching out and disentangling himself. The servants lipped him affectionately, and he returned the gesture as best he could. Despite sleeping on the ground, he felt incredible! Getting up was curiously awkward, and he discovered that his arms and legs were longer. A shadow of hair covered his skin, and his genitals were fully equine. The teen found a stall with a mirror and gaped at the reflection. He was turning into one of the horseboys! Although still mostly human, his body was assuming the proportions of the servants, and his facial features had elongated and pushed out. He was also more mature physically, looking like he might be in his late teens now.
All of this worried him briefly - although he was starting to find the horse servants somewhat attractive, he was pretty sure he did not want to end up looking like them himself. But while he was here, maybe. Memories of last night's erotic adventures brought his impressive equipment to full attention, and he had to masturbate himself with now fully-transformed three-digit hands. A whicker from one of the servants reminded him that he had work to do, and after a jet of semen shot across the bedding, he headed out to his first stall.
Derrick got a lot more physical attention today - last night had apparently broken the ice. Contact with the others was really pleasant, with a lingering sense of warmth where he was touched. During the mid-day break, one of the servants led him over to a mirror. Derrick grinned this time, his reflection no longer recognizable. It was a good thing he was only doing this for a little while longer. The thin coating of hair had thickened into full animal hide, and his face was more beast than human. The biggest different was his head, which was much larger and more rounded than the equine-like skulls of the servants.
The servant beast reached up to rub the dome of Derrick's skull, creating a pleasing warmth that spread down into his brain. The flat fingers pressed downward, and as Derrick stared in amazement, his head started to deflate. There was no pain even though his brain should have been crushed, just the warmth. At the same time, his face shifted more, ears sliding up and brows jutting over eyes that grew larger. Another of the servants came in to rub against his back, and he felt the prod of its thick shaft. Then he gasped as it pushed in, lifting him up. There was purpose beyond pleasure this time, the intense contact accelerating what had been a gradual transformation.
His reflection displayed the rapid changes like time-lapse video documenting the physical transition. His own powerful shaft erupted along with the one inside him, and he whinnied in orgasm. The other servants steadied him afterwards as he stared at the trio of identical horseboys in the mirror. They all looked exactly alike! He could have stood there a long time, but the break ended and he had hay bales to move.
The centaur watched with a smile. With the physical change completed so easily, the mental transformation should take hold even before the third day. By tonight, the former human would find that serving others was all he desired.