Leviathan

Leviathans are aquatic and hostile creatures found in large bodies of water around Shoreline and Subterranean. It is the largest creature in Rain World, sometimes caught killing even vultures. Incredibly resilient to harm, it is not recommended to attack this creature.

The Leviathan is the only freely-moving creature that can be seen eating prey without first bringing it to a creature den.

Trivia (Contains spoilers!)

 * Leviathans share a model shape with the Void Worm.
 * It can eat anything besides its own kind if they get caught in its mouth.
 * In Subterranean, a single, albino Leviathan spawns in a giant pool of water.