Wikisimpsons - The Simpsons Wiki
 Biology and culture
Rigellians are approximately 7 ft. 0 in (2.13 m), continuously drooling saliva, one-eyed beings with eight tentacles. While not on their planet, they wear space helmets, but not space suits. They speak the Rigellian language, which, by coincidence, is identical to English. Their number four looks like a human skull. Peculiarly, they occasionally vomit through their eye, which resembles human crying, but Kodos said they "always secrete ocular fluid at weddings". They must hold hands to exchange long protein strands. They have declared that their religion is to be "Quantum Presbyterians", and that they were present at the beginning of the universe (when it was created five thousand years ago, by God).
 Home world
Rigel 7 is a rocky, desert-like planet with an asteroid field, that orbits the star Rigel. They are the only intelligent form of life on the planet, and their sole preoccupation is assembling a galactic empire for their planet, starting with the conquest of Earth. Kang and Kodos once claimed to live near a ringed planet close to the Earth, implying that it is Uranus.
Years in the future, Rigel 7 is a planet full of lush vegetation and animals where everything can be dangerous. The Rigellians live in peace with the other animals who are adapted to fight wars for some reason. They are led by the Rigellian queen who produces Hillarium for the Rigellians to drink.
Rigellians have access to advanced technology including interstellar travel, ray guns, and the video game "Pong", which in Rigellian society is considered to be cutting-edge. In the future, they seem to downgrade and become a primitive race and live in peace with nature and the animals.
They also have special guns that can impregnate humans.
 Behind the Laughter
In The Simpsons: Hit & Run, it is shown that not all members of the Rigellian species are green. The game's ending features Rigellians of various colors, including red, yellow, blue, pink, red orange, light aqua-green, light blue, dark pink, tan, and orange. However, this is considered non-canon.
The overall appearance of Kang and Kodos is derived from the aliens appearing on the cover of an old EC Comic. They are named after two characters from Star Trek; Kang, a Klingon captain, and Kodos the Executioner, a human villain. Their cook, Serak, derives his name from an inversion of vowels in the name Sarek, the Vulcan father of Spock.
Star Trek would often mention planets in the Rigel system. Rigel VII, the home of Kang and Kodos, is mentioned in the original Star Trek pilot, "The Cage". Rigel IV, to which Kang and Kodos intended to take the Simpson family in "Treehouse of Horror I", is in the episode "Wolf in the Fold".