The name "Jellyfish" is an official name that comes from documents produced by game artist Chuck Jones.
The Jellyfish differs significantly from the Octabrain in its appearance and behavior. The LameDuke version is much darker and has a pink, fully exposed brain. Unlike the Octabrain, it has only two eyes and a beak. Its only attack is to swipe at the player with its front tentacles, yet it deals no damage.
Concept art shows that the Jellyfish was planned to be able to hide under debris and to take control of human remains in order to fire their weapons. None of these concepts appear in the LameDuke prototype.