Void Worm

The Void Worm is a huge creature that resides deep within the Void Sea, underneath the Depths of Rain World. Its silvery body is indescribably long and gargantuan, almost like a massive snake or worm-like creature, and it has a small head with golden eyes and antennae-looking limbs connected to the edge of its long body. It makes 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 swam by.

Appearance (spoilers ahead)
In the Depths, when Slugcat jumps into the void fluid, swimming down long enough causes the 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, Slugcat is left in a pitch-black abyss with only a white light to follow.

Trivia

 * There is a possibility that the Void Worm is related to the Leviathan, although the only proof of this at the moment is the body type and swimming motions, and would have to be officially confirmed by the developers.