The Sea Monster is an unnamed giant octopus that is imprisoned in the zoo of the underwater city Atlantis. It is purple-skinned with two red glowing eyes and many long fangs. It appears as the secondary villain of Eddsworld episode "25ft Under the Seat".
Years ago, Atlantisians captured the giant octopus and imprisoned it in their zoo. Nowadays, Edd, Tom and Matt come to Atlantis to get their keys back, but are accused by stealing them from the museum. The gang masks themselves, fools the police and eventually hides in the zoo. Unfortunately, Tom leans on the glass wall of its cage and breaks it, causing the Monster is free. Its first victim is Tom, whom it angrily sideswiped with a tentacle, making his life flash before his eyes. Meanwhile, Edd and Matt try to escape, but they bump into the cops. The gang is then arrested and sentenced to death by oxygen for taking the key and releasing the octopus.
It is unknown if Atlantisians later capture the Monster and imprison it in the zoo again or if it is still free. But they probably manage to do it, as they did before the events of "25ft Under the Seat".