The brain crab creature is a character who only appears in the episode "Bossy Boots."
This creature appears to be a light red brain with crab claws. It also leaks out liquid/substances at its bottom.
Role in episode
SpongeBob and Pearl are brainstorming ideas of what to rename the Krusty Krab. SpongeBob lists many ideas and then says "The Kreepy Krab," which shows him in a gray and white cloak, holding the brain crab creature.