Aliens are a species of creatures indigenous to planets other than Earth.
In Planet 51, the green humanoids were introduced with antennas and hair on their heads. Notable aliens are Lem, Neera, Eckle, General Grawl, and Glar. The cyclops alien invades the humanoids and destroys the property such as tanks in the Humaniacs movie. The men in this film have snouts resembling the early Whatnots from The Muppet Show and Fraggles from Fraggle Rock. These aliens were invaded by a human called Chuck Baker which was from Earth and especially Rover. Some of the humanoids made later appearences on Escape from Planet Earth as Planet Baab's native species "Baabians". Whereas they have similar designs to their body, leg and arm shapes, but without the antennas, hair and noses and with pants and extra accessories added. They have blue skin, horn-like ears, bumps and the females have fish-like fins on top of their heads. Unlike the glasses for humanoids on Planet 51, they don't have temples.
Other aliens include Doc, Io, Thurman and others who were locked up in the prisons in Area 51. Some of the captured aliens were froze up in the icy prisons which later escaped after the laser ray was accidently broken when they were trying to shoot at Planet Baab while the machine was malfunctionated.