The monsters under Calvin's bed are reocurring pro/antagonists.
First AppearanceIn the monsters' first appearance, Calvin is forcefully sent to bed by his Dad. Calvin asks his Dad if he could check under the bed for monsters, Dad does and finds none, but when asked to check in the dresser, he declines. Guessing that the Monsters are in the dresser, Calvin and Hobbes hatch a plan to battle the monsters. This strip also marks the first Sunday strip and the first mentioning of Calvin's mother.
They vary in appearance, but they nearly all have tentacles. The monsters have tried to eat Calvin on many occasions, but have never succeeded in getting him off the bed. They often use psychological warfare to try to get Calvin off his bed so they can eat him, such as trying to get him to get up and go to the bathroom, or even bribing Hobbes with salmon to toss him over. The only monsters who have been named are the monsters Maurice and Winslow. Both Maurice and Winslow are rather stupid, Maurice having accidentially revealed that there was more than one monster under the bed when Calvin and Hobbes were convinced that there was only one, and Winslow saying that there were no monsters under the bed, then used "we" in reference to the monsters by accident. It is apparent that the monsters are not there all the time, as in several strips Calvin and Hobbes hide beneath the bed. Light is lethal to the monsters.
Watterson has often used monsters as metaphors for his own life. For example, in one strip, Calvin is seen throwing garbage to the monsters because he wants them to leave him alone. This parallels, Waterson's life when he returned from sabbaticals, behind on deadlines and often throwing out mediocre strips that he was not satisfied with.