When grinding on a male SO low that his penis rests on your head.

It's when you lay down flat on you back, face up, with a dildo suctioned to your forehead, and your partner squats down over your head, and inserts the dildo into one of their orafaces, then bounces up and down over you while singing The Irish Rovers, The Unicorn Song.

A non-mythical flying rainbow rhino that poops the rainbow.

Cute fluffy creature with a magical cone on its head that farts rainbows.

The Romans called what we now know as rhinoceros a "unicornus".
Later, they evolved into two species:
1. the white unicorn (of horselike appearance with one beautiful horn on the center of its head)
2. the pink unicorn (of unknown appearance, as it is invisible)

Following in the tradition of "the Dirty Sanchez", this is a sexual act. This can only be done to a girl with hair at least 8 inches long. While she is sleeping you ejaculate in her hair, aiming to the front, specifically. Then, take a small clump of her hair,a t the very front and center, and pull it straight up till it's taut. Rubbing the seminal fluid and holding the hair until it dries, she will wake up with her hair resembling that distinguishing feature of a unicorn.

The act of running around naked with an erection.