Sea People are brine shrimps that made their only appearance to date in "The Simpsons Already Did It".
Background
After having convinced his friends to help purchase the Sea People, Eric Cartman was extremely disappointed as he envisioned them to be more anthropomorphic. At the same time Ms. Choksondik recently died and was found to have semen in her stomach. The boys played a prank on her by putting the Sea People in her coffee and thought that the semen was actually the Sea People.
The Boys sneaked into the morgue and retrieved the semen and thinking it was actually the Sea People, they put the semen in the aquarium. When Cartman awoke the next day, he found the Sea People had advanced into an ancient society.
Having learned that semen advances the Sea People's growth in both society and technology, Cartman acquired ten gallons of semen by going to a sperm bank and a guy named Ralph. Using the semen he acquired, the existing Seaciety, and the new Sea People Tweek purchases, Cartman combined them all in a larger aquarium. Overnight the Sea People advanced into modern civilization, worshiping both Cartman and Tweek as deities.
The internal conflict of having the two boys as deities started a war amongst the Sea People. Soon, people from Cartman's side of the tank started to suicide-bomb the Tweek statue on Tweek's side and vice versa. After several rounds of suicide bombing, both sides destroyed each other and the fish tank with nuclear weapons.
Appearance
As they appear normally, Sea People look like brine shrimps. As their society progresses with the help of semen, they acquire clothes, most notably the white toga with blue and red stripes.
As they appear in Cartman's vision, Sea People are anthropomorphic Brine Shrimps all of which wear various swim wear.