Dan was feeling a strange swelling from the sides of his head, then a new weight that wasn't there before. He realized something was wrong and reached up there. He encountered a couple of bone-like masses that definitely didn't belong! He raced into the bathroom to look in the mirror and was shocked to discover a pair of curved, pointed growths blossoming from his skull. It was still going... they got longer and longer... until finally they stopped.
"They're... they're..."
He couldn't bring himself to say it, but they were horns. Off-white horns that curved up and ended in dark points, just like what you might see on a bull's head. He locked his hands tight around them and tried to pull them out with all his strength, but they were solidly planted in his skull, so he just fondled around up there and explored them a little bit.
"Kat! Kat! Kat!" he called out in a panic. In barely a second, his sister raced in the door. She took one look at Dan and his reflection (and the startled look on his face) and burst out laughing.
"Well, Dan, I knew you were always horny before, but this takes it to new heights!"
He turned around with a grimace and complained. "This isn't funny, Kat. Do you realize what's happening to me?"
"Well, yeah, I mean you're turning into a beast. This is the best birthday gift ever! The first spell I've tried on another person, and it's actually happening!"
"But, but, but..."
"Oh don't make such a fuss, Dan. Your own unconscious chose the animal. Just go back to whatever it is you do up here, and we'll see how your changes are coming along later."
Then she stepped out the door and was gone. Dan tried to follow her, but his horns slammed into the doorway. When he finally maneuvered himself back out into the hallway, she was gone.