Sheldon considers Marvin stupid. Marvin is notably smaller than some of Sheldon's other cousins.
Cousin Marvin briefly appeared in the short, "Plankton. A Thank You Would Be Nice" when Plankton was explaining how plankton are the most important living creatures on the planet. He uses cousin Marvin, who he describes as stupid, as an example, but he then explains how Marvin has planktonic plant brains that make up half of the Earth's oxygen.