"A safety deposit box?"
"For this." His hand thrust out, right through the skin of Derrick's crotch, and started digging around. Grinning, the boy carefully extracted Derrick's entire erect penis. Holding it up like a prize, he ran his other hand down the length to remove the pubic hair still clinging to the other end.
"You're uncircumcised, huh? Very cool. Let me just stow this away."
He dropped it into the box, closed the lid, and locked it. "We'll hold onto this. Now then, it's time for your treatment."
Derrick was nervous, but his curiosity was piqued. "Treatment?"
"Mares aren't born, D. Here, we make em'. And if you're going to be a proper mare, we need to give you some new genitals."
He started rubbing the bare area where Derrick's cock used to be, the soft feel of his hands triggering some kind of strange ecstatic feeling in his mind. Whatever inhibitions he had about boys touching his crotch began to fade away as he took notice of just how handsome the stable boy was. His piercing eyes, his perfectly-combed hair, the sharpness of his features.
His crotch began to melt and glop together like wet putty as the stable boy's experienced hands began forming the outer structure of a functioning vagina. First, the sensitive clitoris so Derrick could receive pleasure, then the outer labia to give his new lady purse some shape. Eventually, his fingers began to slip in, causing the vagina to moisten and release a little bit of fluid as he manipulated it.
Then, he suddenly moved his hands up to Derrick's chest, gathering big handfuls of his flesh and squishing it together, forming big balls with his nipples on the end. Derrick realized he now had generously sized B-cup breasts.
Everything started coming together at once. Derrick had never been a particularly masculine figure, and his small body was always thin and his face soft and largely free of facial hair. He always liked his hair short, in the style of a pixie cut because it was easy to manage. Now that he had boobs and a vagina, he saw that he was now naked. The stable boy was looking at him with a lustful gaze. He'd unlocked the safety deposit box and retrieved the penis.
"I have to run a quick quality control test on you, love. And I'll be fucking your beautiful ass with this."
Pulling his pants down to reveal a bare patch where his genitals should have been, he worked Derrick's cock into his body. Once it was in place, he stroked it for a few seconds until it became hard. Derrick noticed that two testicles had suddenly popped out in a sac beneath his new foreskin.
"Come here, girl!"
Derrick jumped right on top of him, and slammed his cock right inside of her vagina. She wrapped her legs around him for support, and he proceeded to fuck her until he forced her to the ground and put the tip of his penis in her mouth. A massive load of semen came shooting through his cock right into her throat. She swallowed every last drop of it. "Your cum, it's so creamy and salty at the same time."
"Heh. You know, that old stallion can have some other gelding to stick his pecker in. I rather like you, sweetheart. But the choice is up to you. You want to be my girl or a mare?"