The Monster (also known as Blackhole Green Monster) is the main antagonist in the Regular Show: Pilot. It shows up when Mordecai and Rigby have 100 ties while playing "Rock, Paper, Scissors.
Since no one could decide which one of them would get the chair and they have 100 ties, he decides to eat the chair by sucking it up into a dark black hole. He is defeated when they decide to end it with a 101st game of "Rock, Paper, Scissors." The name Rochambeau is a play on the phrase "Ro Sham Bo", which is another name for the game of Rock, Paper, Scissors (Ro stands for Rock, Sham stands for Scissors, Bo stands for Paper). Rigby stuck his head into the monster to get the chair. They ended up only getting half the chair. The other half went to the monster. The Monster shows up in First Day as well, however it's only due to the fact that it is an extended version of the Pilot.