The Sandwich Monster is a minor character in the Season 6 episode Lunch Break, he appears in an imaginary story Mordecai and Rigby make up while trying to finnish a ten-foot-sub by the end of the day.
In the story, they are disgusted from eating nutrition cubes that are the only thing that they can find since the earth blew up. Then a collision occurs and Rigby is sent to investigate after inevitably losing in punchies, where he finds truck of sandwiches to eat. They microwave what they find and try to eat it.
But when Mordecai tries to cut it, the sandwich reveals itself to be alive and they chase after it, through the Park and house, a mountain, a stage, until they reach a rooftop. But then the sandwich is catapulted to another rooftop by a spatula from the sky, and then the spatula swats Mordecai and Rigby into the ocean. through space, and into a building where they meet RGB2.