Elephant fish
A Sea Moyster is a monster who appeared in the episode "Prehibernation Week". When Sandy ordered the other people in town to go look for SpongeBob she told the other people check in this here moysted cave. But the moysted cave is miskaten to be revealed as a elephant like monster with three eyes and a long trunk.
Trivia:
Due to having a trunk like an elephant , Sea Moyster could be an elephantfish, or possibly a spoof of it.