When you have awoken from a one night stand and have realized you have just had sex with a rat-like being and decide to sprint out of the room.
"I woke up after hitting this chick, turns out it was some swamp donkey so I pulled a Rapidash"
A very good substitution to other words when trying to show happiness, or agreeing to something. Usually said by poke-nerds of all sorts.
Girl#1 - FUCK YEAR!
Girl#2 - FUCK YEAH, RAPIDASH.