An uncommon sexual act whereby one person kneels naked on their hands and knees (doggy style). The other person takes their penis and ejaculates all over the kneeling person's back, being careful to leave a sticky trail down the other person's spine. Then, the standing person takes an electric razor and shaves part of their pubic hair. Finally, taking a handful of hair, they sprinkle it along the sploogy trail where it sticks, forming a mane.
This is often used as an initiation practice in college fraternities. The "pony" created would then have to canter around campus in the middle of the night, making horse noises and waking up some of the dorm residents, until quickly spirited away by the pony creator, before the law arrives.
note: definition courtesy Gabe.