Cthulu

"Zim knows that when he made Cthulu, a blob monster that once absorbed energy, it was the reason why the Vortians and Irkens were no longer friends to this day."

--Su Ji-Hoon, Dirty Little Secret

Cthulu, also known as the Infinite Energy Absorbing Blob, would have made his appearance in an Invader Zim episode, "The Trial." He was an "infinite energy-absorbing thingy" created by Zim while he was a scientist working on Planet Vort. Unfortunately, he ended up eating Tallest Miyuki, and later Tallest Spork when he came back to retrieve his collar.

Not much is known about him, but the title "Stage 1" on his concept sketch would imply that he later transforms into another form. Perhaps the "unspeakable, now enormous" form mentioned in the script for The Trial would have had a different appearance, or it could simply be that his design was in the early stages of development.