These robots have appeared in almost all of the Mega Man game series, except for the Mega Man Legends series. They mostly appear as enemies, but there are also bosses such as the giant Met in Dr. Wily’s castle in Mega Man 4, Metall Daddy. In Mr. X’s castle in Mega Man 6 there is a Met boss operating a tank, named Mettonger Z. In Mega Man ZX, there is a giant Met miniboss, named Powmettaur.
Info from: The Mega Man Knowledge Base