Metals is an android built by Dr. Gero. Metals is the only creation that Gero has made that serves only him. But after Gero was killed Metals didn't have anyone to take orders from and he went Haywire. He caused havok for the 5th year of peace after the Cell saga. Metals was the first opponent Goten had faced and Goten finished him off with an Oozaru punch through the chest, just how Goku beat King Piccolo when he was a child.