Leviathan

Leviathans are aquatic, hostile creatures found in large bodies of water in Shoreline and Subterranean. They are the largest predators in Rain World, and hunt many large creatures up to and including Vultures. Leviathans are biomechanical, killing prey with powerful mechanical, 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 or good mastery of swimming techniques. It is advised to either avoid leviathans completely, or to attempt to ride a Jetfish around them - additionnally, it can also be safe to pass over them in shallow waters where they cannot turn around. 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 can 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 can be killed, their resistance is beyond what players will typically be able to overcome 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 is obviously not recommended, as it would take an entire cycle and only prevent the Leviathan from appearing until it naturally respawns a few cycles later.
 * 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, despite being so large, can be eaten by Monster Kelp, though it is unlikely they would come close enough to each other outside of arena mode.
 * The metal plate asset on the front of the leviathan is reused in-game on the upper bodies of squidcadas.

Speculation

 * The lake in Subterranean has an albino Leviathan and, as Hunter, 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.[citation needed]
 * It is 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 is extremely unlikely a Leviathan would be able to reproduce and yield offspring that will 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.
 * Leviathans may be a physical-world representation of the Void Worm, created through genetics by the Ancients. This may be to revere the Void Worm that the Echoes may have told the other Ancients about, and may explain why they are massive, marine, near-immortal creatures, and basically the closest thing the world has to a Void Worm outside of the Void Sea.