Void Worm

Void Worms are huge creatures that reside deep within the Void Sea, underneath the Depths of Rain World. Their silvery bodies are indescribably long and gargantuan, almost like massive snake- or worm-like creatures, and they have small heads with golden eyes and antennae-looking limbs connected to the edge of their long bodies. They make an appearance in the endings for all three difficulties. Although the player only encounters one void worm that actively interacts with it, there are many others that can be seen and swum by.

Appearance (spoilers ahead)
In the Depths, when the slugcat jumps into the void fluid, swimming down long enough causes a void worm to appear. After a few moments of examination, it "hooks" Slugcat with a string, and then pulls slugcat upwards, then back to the depths of the Void Sea before releasing slugcat and vanishing. Upon being released, the slugcat is left in a pitch-black abyss with only a white light to follow.

Trivia

 * There is a possibility that void worms are related to the Leviathan, although the only suggestion of this at the moment is the body type and swimming motions, and so this would have to be officially confirmed by the developers.
 * If the main portion of a void worm swims near the slugcat, a loud rumbling will be heard, and the slugcat will be forcefully moved in the direction the worm was moving.