The Bog Monster is a fictional character that Calvin and Hobbes read a horror story about. The monster and the book it was in, "The Hideous Monster That Waits in Your Backyard to Dismember You", were mentioned in a single strip where Calvin and Hobbes went camping in their yard.
Calvin and Hobbes were reading the story and were spooked by it, while they were interupted by a branch loudly snapping. They then quickly turned out the lights and lied still, but it started to unzip the tent. They then pulled down the tent posts while blinding it with a flashlight, then dashed back to the house. Calvin was next seen traumatically siting on his mother's lap while she asked where his Dad was. Then a shout was heard "Where's that kid?!" implying that the "Bog Monster" was just Calvin's Dad.