He raced to the bathroom, as he felt a ballooning in front in a lustful and "have-to-really-piss" sort of way.
Upon entering the bathroom, he immediately dropped his shorts, and a long, hoselike appendage spilled out. The girth of a beer can, and the length of a meter stick! It was just finished turning black up and down it's length, as the tip flared out like the penis of a jackass. Then his balls began to inflate and get heavier. They, too, turned black and the skin became leathery.
Gasping at his new monstrosity between his legs, he needed to go, and go now! He tried to grab the oversized appendage in one hand, he it was so thick, heavy, and long! Suddenly, he lost all control of his bladder as it truly became a hose.
The piss he was spraying everywhere was so yellow, it was almost greenish. He just stood there, with his right hand gripping the base of it, totally in shock and feeling the relief. The thing sprayed like a firehose for a full minute or more. The relief was so unbearable, the thing stood at attention and started throbbing. It started pumping itself up and down. His balls churning overtime.
Then he felt it like an insatiable lump at the very base of his new donkey cock. He felt it rush up through the entire meter-long tube of flesh. His inhuman tip flared outward, and yellowish-white jackass seed gushed out in long, ropey globs.
Through the process, Dan began drooling as fireworks exploded throughout his brain. After a half dozen pumps or so, it finally subsided. Looking around, he had made a hell of a mess! It was completely uncontrollable.
Sickened, he started to cry, as he tried to grip it again in his puny little human hand. Puny and little, in comparison to this new appendage. And it was soooo....HEAVY! Sniffling, he grabbed one of his new black balls in his hand. It was the size of a tennis ball, but felt like a heavy lead paperweight wrapped in a black, silky, velvety pouch.
"How will I ever hide this thing?" He thought.
"You won't," he heard his sister's voice behind him.
"You'll have that....."THING".... for a while as well. And it'll always be attached to you, and flapping wide open for the world to see. Your tall pointy ears pointing upright, your long pointy tail pointed downward in one direction behind you, and that black hose of yours pointing downward in the other direction. Those are the prominent features of a fully-gown adult male donkey."
"Fully grown adult?" he sniffled.
"That's right, little brother. You're going to also lose your youth. You'll be a fully grown adult male donkey!"
Disgusted, he pushed his sister aside and raced to his room. Well, "waddled," more like it, as his huge dick and balls would allow him to no longer "race" ever again. At least, not as a human boy. The mess he left behind already forgotten.