Not SpongeBob briefly appeared in a flashback in "Truth or Square," in which Squidward was reflecting on how happy he 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 eyes, a black beard, olive green lips and olive green limbs, and an orange dorsal fin. He wears an grease stained apron, light blue pants, and a Krusty Krab hat.
In "Truth of 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 remembered him that way to show that he was not SpongeBob.
The idea of characters having the word "not" in the name was used later for the character, Patrick Not-Star.