Mr. Monster is a green monster who manifested inside of Cow's closet in the episode Monster in the Closet. Mr. Monster can only be seen by certain individuals at certain times. Originally he was friends with Cow, but her parents forced her to put him back in the closet for being a monster. Afterwards he briefly moved down to the living hallway closet, and when Cow went to check her closet, he was gone, making her doubt he ever existed and completely forget about him, much to Mr. Monster's sadness. Chicken, who was now capable of seeing him, then invited Mr. Monster to go bowling with him and the two became good friends.
- His cousin is a warthog.