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 only makes an appearance in the endings for all three difficulties.

Appearance (spoilers ahead)
In the Depths, when slugcat jumps into the void fluid, swimming down long enough causes the Voidworm to appear. After examining you, it seems to "hook" you on a string, and then goes extremely fast downwards. This repeats a few times, and then it leaves. The area it leaves you in is completely black.