"I'd like to be a centaur!" Derrick flushed and looked down at the water. "I mean, if that's not too much to ask. That's way more than a change of hair color or race."
Hebe laughed and tousled his hair. "How wonderful! You are precious, and just perfect to be a centaur colt! They are very popular with guest children. Would you mind spending your vacation time living with a centaur family? I am sure your parents will not mind."
"That would be even better!" Derrick was delighted. "I can see what it is like to be a real centaur!"
"Definitely!" Hebe looked back towards the entrance and called out "Caliopa! Send for Sasha. Tell her we have a willing foal." The receptionist stuck her head in and looked surprised when she saw Derrick. "Oh, my! He is a real cutie! Sasha will be thrilled!" Then she left.
Hebe turned back to Derrick. "Now then, let's get the basics taken care of. Stand up for me, little man."
Derrick got up awkwardly, helped by Hebe, and then stood in the knee-high water. He felt a little odd, uncoordinated, but also very energetic. Then he shivered as Hebe's warm hands slid down from his buttocks and started to massage his legs. Nothing happened at first, but then his muscles started to pull and a dull ache started in his hips. "Ow!"
"Sorry. This may hurt a little, but I promise it will not be bad or for long." Hebe kept working, and Derrick forgot the pain when he looked down and saw that he was swelling below the waist. Twisting around, he watched in awe as his rump expanded back, thighs getting deeper, legs getting longer. He gasped when each leg pulled apart into a foreleg and hind leg, though it did not hurt and there was no blood. He had to shift as his lower body lengthened, very wobbly until the four limbs took on the familiar look of a horse's legs. The ache was gone, replaced by a strong itching sensation as dark hair spread rapidly over his skin, and a very odd twist of muscles over his rump as a tail formed. Derrick stared in wonder as Hebe ran her hands over the sleek horsehide of his now equine lower body.
"Ah, Hebe!" He looked towards the entrance to see a female centaur beaming at him. She was very large, with glossy golden brown horsehide and white-blonde mane, tail, and hair on her head. She was also naked, as most of the mythical beings were in the Spa. "He will be my foal?"
"Oh, we have some personalization to do, but yes. He specifically asked to be a centaur colt." Hebe nodded to the centauress. "Derrick, this is Sasha. She will be your new mother while you are here. Sasha, this is your new foal. What name and age would you like to give him?"