The Sludge Monster is a creature made of sludge.
In the comic strip, Garfield watches a movie about the Sludge Monster, which made him paranoid for a week. Later, he, Jon and Liz saw The Sludge Monster Meets Vermin Man at a drive-in theater, with Garfield rooting for the Sludge Monster. It was the ninth time Jon and him had watched the film.
In Garfield and Friends, it was a local folk tale that Jon used to scare Garfield and Odie while they were staying at a hotel. When the three of them leave (due to Garfield and Odie's antics), the sludge monster turns out to be an invisible friend of the hotel clerk named Maurice.