Amoeba
Character
Amoebas are microscopic one-celled organisms that were first shown in the episode Pretty Patties when Mr. Krabs wasn't receiving business in the Krusty Krab. They also appeared when SpongeBob turned Plankton into a baby (as seen in the episode Goo Goo Gas). He was shrunken down after being hit by the Goo Goo Gas to microscopic small-ness. The Amoebas were shown to say that Plankton's baby size is microscopic.
Plankton would later get a pet amoeba in Plankton's Pet and name him Spot.