Aliens are a diverse race of antagonistic creatures that originate from worlds beyond that of the main setting; they can be extraplanetary and extragalactic in origin. Even some humans who are not native to Earth but are virtually identical to Earth humans in every way and are native to the worlds beyond other than said planet can be classified as aliens.

If they come from alternate dimensions or before existence itself (e.g. Minions, Bill Cipher, It, Invictus, Mind Flayer, King Ghidorah (anime), GOLB, Shadowman, The Darkness, and Dimentio) they do not count.