This Lake Monster is a monster in the Camp Lazlo episode, Gone Fishin' (Sort of).
It appears to be a serpentine creature with purple skin covered with dark purple polka dots and a horse like head with teeth sticking out of its mouth, antennas, and three mandibles located around its head.
The creature appears near the end of the episode where it rises out of the water and starts attacking Scoutmaster Lumpus. Lumpus tried to run away from the monster but was caught by its jaws along with Lazlo. In the post credits, it is revealed that the creature could talk and is really a vegetarian after Chip and Skip received its snow globes back to it.