Not SpongeBob briefly appears in a flashback in the episode, "Truth or Square," when Squidward is reflecting on how happy his life was before SpongeBob moved in. He used to work at the Krusty Krab after Jim resigned, but before SpongeBob gained the position as a fry cook.
Not SpongeBob is a dark green fish with large light yellow eyes, a black beard, olive green lips, olive green limbs, and an orange dorsal fin. He wears a grease stained apron, light blue pants, and a Krusty Krab hat.
In "Truth or Square," Not SpongeBob is seen cooking Krabby Patties until he is interrupted by Squidward, who picks him up and kisses him on the forehead, then drops him.
It is unknown if Not SpongeBob is his real name, or if Squidward just remembers him that way to show that he is not SpongeBob.
The idea of characters having the word "not" in the name was used later for the character Patrick Not-Star.