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 also seem to have a maximum amount of food they will eat 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 (Spoilers

 * 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.
 * In Subterranean, a single, albino Leviathan spawns in a giant pool of water. This is a case of albinism that has developed in the creature over time, not inherited since birth, which has been documented in cave-dwelling fish who become albino in an extreme lack of sunlight, similarly to how the Leviathan got stuck in the cave through unknown means.
 * The lake in Subterranean has an albino Leviathan and two albino Jetfish living in it. The Leviathan, Jetfish, and lake itself bear many similarities to the Void Sea. The Leviathan represents a Void Worm, especially in color, and the Jetfish resemble the Void Spawn present in Depths. The structure of the lake, with spikes protruding from the sides, is exactly like the structure of the room housing the entrance to the Void Sea. This is compounded by the developers confirming the Leviathan's similarity to Void Worms is not a coincidence.
 * 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.
 * It's most likely that Leviathans are biomechanical organisms creatures with mechanical augmentations designed by the previous inhabitants of the world, possibly as defense for the structure hidden within the shoreline. Since it's extremely unlikely a Leviathan would be able to reproduce and yield offspring that will be able to have the same mechanical parts, the Leviathans we see in game are almost certainly the same that were alive during the time of the Ancients.
 * 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 absurdly humongous stockpile of explosives or a modded weapon, but doing so simply makes the leviathan stop moving, and does not yield any reward.
 * When killed in a Hunter run, a Leviathan is worth 25 points.