The Cyberdemon in Doom 3

The Cyberdemon is the most powerful and infamous monster in the Doom series. As his name suggests, he is a large goat-like Demon with robotic parts, such as a giant gun for a hand. He is extremely difficult to defeat, and is known as one of the hardest video game enemies ever. One magazine infamously gave this as a strategy: "To defeat the Cyberdemon, shoot at it until it dies."

The Cyberdemon (pre-Doom 3)

He appears in the original Doom as the boss at the end of the second episode, and in Doom II he makes multiple appearances. In Doom 3 as well as Doom RPG, the Cyberdemon is the final boss. He also appears in Wolfenstein RPG, in a fully organic form known as the "Harbinger of Doom". When defeated, his arm and leg (the parts that are robotic in the Doom games) fall off and he says he will return in the future to take revenge on B.J Blazkowicz's descendants, thus implying that Wolfenstein and Doom take place in the same continuity.