The Boogey monster is a blue monster with 3 tails. The Klayman consider him to be an ancient legend, and are shocked to find that he exists.
the monster only appeared in the episode "Boogey Monster", where he shows up and slaughters the Klayman, and in one of Knox's "Forgoten episodes" known as "Boogey Monster 2" where the army tries to kill the Boogey Monster.
Oddly enough, the Boogey Man, being such an underplayed character, has a striking ammount of history behind him.
According to one of Knox's commentaries, the Boogey Monster was meant to be a villan in a movie where a group of friends was attacked by these creatures in the woods. Knox decided to scrap the project and is apparently glad that he did, but kept the monster models, and made one of their eyes "goofier" and this became the boogey monster.
In a more recent commentary, Knox revealed that the Boogey Monster was one of his favorite characters, and he would like to bring him back. Sadly, the original Boogy Monster has become so old and brittle, animation would become nearly impossible with him.