The Demon in the Mattress is a demonic spirit personified as a possessed green mattress and a villain from Courage the Cowardly Dog. He makes his presence known by manifesting a face to frighten Courage. He later takes possession of Muriel changing her appearance to green skin with wavy orange hair with a deep, male, demonic voice. Courage and Eustace spend most of the episode to get rid of the evil spirit.
Until Courage uses the incantation to drive the spirit out of Muriel. Minutes later, he possesses Eustace. Muriel knocks him out and orders the mattress to be taken back.