Alien

Aliens (宇宙人), also referred to as extraterrestrial life-forms (地球外生命体), are theorized life-forms which do not originate from planet Earth. Although humanity has yet to encounter extra-terrestrial life, aliens feature prominently in science fiction, conspiracy stories and other parts of popular culture.

Although no truly extraterrestrial life-forms have been canonically confirmed within the Toaru series, aliens and various things associated with them have been referenced numerous times within the story.

Baggage City Arc
One of Kihara Byouri's Dark Matter-based transformations, Reference: Little Grey, was based on a alien.

Coronzon Arc
One of the Crowley's Hazards involved in the invasion of London had the appearance of an alien with an oversized head.

Toaru Kagaku no Railgun SS: Liberal Arts City
The Laveze Squadron involved in the Liberal Arts City incident were named after aircraft from the movie "Alien Wars" modified to fight UFOs.

Trivia

 * According to Haimura Kiyotaka, he designed Aleister Crowley to be something "like an alien in human skin."