Cyberdemon

The Cyberdemon is the most powerful and (in)famous monster in the Doom series. As it's name suggests, it is a large goat-like demon with robotic parts, such as a giant gun for a hand. It 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."

It appears in the original Doom as the boss at the end of the second world, and in Doom II it makes multiple appearances. In Doom 3 as well as Doom RPG, the Cyberdemon is the final boss. It also appears in Wolfenstein RPG, in fully organic form as the "Harbinger of Doom". When defeated, it's arm and leg (the parts that are robotic in the Doom games) and says it will return in the future to take revenge on your descendant, thus implying that Wolfenstein and Doom take place in the same continuity.