"A missile-launching skyscraper with goat legs. 'Nuff said."
-Doom II instruction manual
Technically speaking, the Demons of doom are not Aliens; However, due to their demonic-pilgrim like behavior of assimilating earth's culture, they do not behave like typical demons either despite being from biblical hell. It could be postulated that after an eternity of punishing bad folks, they want something fresh and new.
The Cyberdemon, along with the Spidermind, is a very large cybernetic demon that acted as one of the Generals in charge of leading the invasion of Mars' moons, Phobos and Deimos. These monstrosities are extremely powerful and seem to have been created through the use of horrifying genetic, cyber, and biological manipulations to their physical form. The appearance of the Cyberdemons is rather menacing, they are very tall, have a rocket launcher surgically attached to their left arm, a robotic right leg, a wired midsection, and a muscular upper body.