Nightmare Membrane

Nightmare Membrane only appeared in one episode, "Halloween Spectacular of Spooky Doom". He, along with Nightmare Gaz, take Dib to Nightmare Bitters. He has a skull-like face, extendable eyes on stalks and clawed fingers. Apparently he can materialize from nothing, and his robe can even sprout tentacles. Nightmare Membrane shares several similarities to ghosts, as he doesn't seem solid and has the ability to warp himself in ways that a living creature could not. He also appears to glide, rather than actually walking. 

Seeing as the nightmare realm is inside of Dib's head, it is theorized that Nightmare Membrane is a representation of how he sees Professor Membrane at his worst. The same may be said for nightmare Gaz.

Not much else is known about him.

He was voiced by Rodger Bumpass.