Leviathan

Leviathans are aquatic, hostile creatures found in large bodies of water around Shoreline and Subterranean. They are the largest predators in Rain World, and they are incredibly fearsome beasts, hunting many large creatures up to and including Vultures. Leviathans are as mysterious as they are terrifying; they are biomechanical, killing prey with powerful hydraulic, clamping mouths, but possessing bodies that look organic with long, thin fins. Leviathans can outpace a swimming slugcat on their own, and outmaneuvering them is near-impossible in open waters without aid. It is advised to either avoid the leviathans completely, or to attempt to ride a Jetfish around them. Leviathans seem to have a near-perfect sense of the creatures around them, not relying on sight or sound alone. Leviathans only eat a certain amount of food in a cycle, meaning it may be viable to feed large prey such as Lizards to a Leviathan, or to simply wait until the leviathan has caught sufficient prey on its own, in order to pass safely.

Trivia

 * Leviathans can eat any creature that gets stuck in its mouth, no matter how large, because any body segment caught in its jaws will delete the whole creature. This includes other Leviathans, but it is highly unlikely for this to happen.
 * Leviathans are stated in the devlog on Tigsource to be designed to invoke the fear of having a piece of seaweed touching your feet in the ocean.
 * Although Leviathans have a health value, it is beyond what players will typically be able to deal in a single cycle, making them near-immortal. A persistent player may be able to kill a Leviathan with an incredibly large stockpile of explosives or use of modding, but doing so simply makes the Leviathan stop moving, and does not yield any reward.
 * In Subterranean, a single, albino Leviathan spawns in a giant pool of water. This is similar to the albinism documented in cave-dwelling fish who become albino in an extreme lack of sunlight.
 * Leviathans are, without a doubt, the largest creature in Rain World, with the exception of the Void Worm, which has no real effect in everyday gameplay. Even the feared Vultures can be devoured whole by a Leviathan, and likely even Daddy Long Legs, although the two would never have interactions with one another.