From Monkey Island wiki
The Sea Creatures are an unnamed, ancient, mythical species of aquatic life. They serve an important role in finding the legendary sea sponge known as La Esponja Grande, a secret kept by the Vaycaylians of Spinner Cay. Their only known natural enemy is the manatee, who enjoys eating the creatures.
Appearance and Biology
The creatures are larger than most human beings. They have a shell and body similar to that of a turtle, but have a head resembling a seahorse's. They also have large antennae on the top of their head, with a significant bulb at the end. The creatures have a large fin at the back of their body for propulsion, with two smaller fins at the front of their body.
The creatures' skin appears to be mostly light-green, with purple colouring around their beak and eyes.
The creatures are known to have an internal skeletal system, with skulls and other bones being found on ocean floors around the Jerkbait Islands.
Role in the game
In "The Siege of Spinner Cay", the second episode of Tales of Monkey Island, Guybrush Threepwood is able to summon the creatures by combining the ancient Summoning Artifacts, speaking the sacred summoning words, and throwing the combined summoning items into the sea. Three Sea Creatures then appear out of the water, and then lead Guybrush and Winslow on a course towards the legendary La Esponja Grande.
In the third episode, "Lair of the Leviathan", Bugeye leaves the head of a dead sea creature near Guybrush's ship as a warning not to cross the Democratically United Brotherhood of the Manatee Interior. Guybrush's (and Winslow's) finding of the head simultaneously makes a reference to the movie The Godfather while Guybrush's yell "Seabiscuit!" (which, as it turns out, is because he finds an actual seabiscuit) is a reference to the film Seabiscuit. At the end of the episode The Howler Monkey, the ship of the recently reunited Coronado De Cava and Bugeye, is swallowed (again) by a giant manatee when Guybrush launches the Sea Creature's head at it.