The monster fish makes its first appearance in "Mystery Dungeon." It lived inside the dungeon in hot water where Ice King, Neptr, Lemongrab, Shelby, and Tree Trunks were trapped. Shelby soon has an idea to use Tree Trunks as bait for the monster which causes the monster to consume her. The gang then pulls the monster out of the water, and Tree Trunks cuts her way out of its belly. The pool then drains and the gang escapes into the next room. Later in the episode, Ice King reveals that he planned for Shelby to be bait.
The Sea Monster has a large blue body, beady red eyes, and numerous sharp, skinny teeth.