In the "Pilot" and "First Day", the Black-Hole Monster is unintentionally summoned by Mordecai and Rigby, and tries to grab the couch they are betting on. With all of its power, it sucks in everything in its path, bringing it into the black hole. Mordecai and Rigby are being dragged towards the monster while holding onto each other's ankles, and onto the couch. Mordecai convinces Rigby that he's going to let Rigby win the bet/game. Rigby doesn't believe him at first, but when Mordecai breaks the tie, the monster disappears along with the black hole.
The Black-Hole Monster is a hairy, green monster, with yellow eyes and large pupils. It bares a strong resemblance to the Grinch.
- It resembles the Grinch from "How the Grinch Stole Christmas".