The Bog Monster was a one-eyed, three-toed nocturnal creature native to Preplanis that fed on electrical power. The bog that it lived in had properties which rendered the creature invisible. ("Ghost in Space")
The Bog Monster was not given a name in the episode (Smith thought it was his uncle Thaddeus), but this is the name given it in the Internet Movie Database. It was played by Dawson Palmer in an uncredited role.
When the creature is invisible, its footprints are three toed. However, when the creature materializes, there are more than three toes and each toe has a large claw turned and pointed downward.
The Monster was possibly a mutated member of the Cyclops' species.