Void Worm

The Void Worm is a huge creature that resides in the Void Fluid. Its body is incredibly long, almost like a big worm, and it has a small head with golden eyes and antennae-looking limbs connected to the edge of its long body. It can be seen in the ending.

In-game (Oh crap ultimatopotato spoilers)
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.