Amoeba

Amoeba are microscopic one-celled organisms that were first shown in 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 Goo Goo Gas). He was shrunken down after being hit by the Goo Goo Gas to microscopic smallness. The Amoeba was shown to compare Plankton's baby size is microscopic.

Plankton would later get a pet amoeba in Plankton's Pet and name him Spot.