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 smallness. The Amoebas were shown to say that Plankton's baby size is microscopic.
Plankton also gets a pet amoeba in Plankton's Pet and names him Spot.