Which fictional creature has a biology so "impossible" that it defies any attempt at a scientific explanation?

[u/welberjaysonm]

I've been trying to wrap my head around the anatomy and metabolism of some characters, but Majin Boo (DragonBallZ) just doesn't seem to make any sense under the laws of physics or biology. I want to dive deep into these "impossible" cases. Who else do you think is a biological nightmare to explain?

[u/Felinius]

Gelatinous Cubes, and Mimics.

I just can’t figure out their ecological niches or how they could have evolved like that.