Calvin and Hobbes prepare to kill a monster.
Calvin has a wide variety of imaginary monsters living under his bed. 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. 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.
Meaning
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.
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Alter egos
Both Calvin and Hobbes have had monsters as alter egos.
Calvin monster
In a single weekday strip, Calvin appeared as a hirsute monster alter ego as an allegory to his unruly haircut.
Hobbes monster
Hobbes also appeared as a monster alter ego in among the Transmogrifier Gun Alter Egos of the first Transmogrifier Gun story arc (2/8 through 2/20 1988). Specifically, it originated in the Sunday strip in which Calvin fought with Hobbes over the Transmogrifier Gun.
The monster was the last of Hobbes' Transmogrifier Gun Alter Egos; Hobbes was returned to his normal form following the Sunday strip in which he first appeared.