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.