Metallus first appeared in an episode of the 1960's Space Ghost cartoon titled: "Space Armada" in this episode Metallus constructs missiles with appendages that he calls his, "Metal Men" he then uses these missiles to try and destroy Space Ghost and his two side-kicks, Jan and Jayce. Space Ghost Defeats Metallus, thus thwarting his nefarious plans. Metallus later joined a team of supervillains called the Council of Doom. This consisted of 5 other Space Ghost enemies, including Brak, Spider-Woman, Creature King, Zorak and Moltar.
Metallus Backstory: Not much is known about Metallus' history but in one episode of Space Ghost titled: "The Robot Master" Blip (Jan and jayce's pet monkey) removes Metallus' helmet to reveal that nothing is underneath, this shows that Metallus is most likely a robot. Although this is shown to be just a decoy Metallus used as a diversion so the real Metallus could escape, it was probably just an even more advanced diversion wherein Metallus allows Blip to blow him up, knowing that he could be rebuilt so that Space Ghost would only take in a false Metallus. And though it may seem a bit of a stretch to think he would blow himself up simply to throw off Space Ghost, it actually makes sense as it would be favorable over simply blowing up another decoy and then escaping as there would still be a risk of being caught. He could possibly have been created by the Creator of The Metallo Hawks who appeared in the episode called: "Space Birds" because of The creator's robotic expertise. Another thing to suggest that the creator of the Metallo Hawks created Metallus is that at the end of the episode, "Space Birds" Blip repeatedly slams The Creator's head in an entry hatch of a ship, this would have really hurt him, and he likely died of his injuries later on. This could be why he created Metallus, if he knew he didn't have much time left, he might have created Metallus to carry on his scientific genius and take revenge on Space Ghost. Metallus also used robotic falcons very similar to that of the Metallo Hawks in a Comico One shot titled, "Space Ghost #1"
*Please note: This page mentions that Metallus first appeared in 1997's Cartoon Network Presents Space Ghost #1, this is untrue, I believe his first comic book appearance was in the Comico one-shot I mentioned earlier but I'm not certain, he could have appeared in comic books earlier than that. He also appeared long before that, on the small screen.