Metallicus was created by Mitchell Chadwick Robotnik, Dr. Eggman's father, twenty years before the events of Sonic the Hedgehog (1991). He was the first robot to be powered by an animal in its interior (which later inspired Eggman himself to build his early robots). But two years later, Mitchell got infected by the mysterious, yet menacing Blue Moon Virus. As he died, he told his son to store Metallicus in his lab and run far, far away. Eggman did so, but he went mad and made blueprints for animal-powered robots, such as Motobugs, Caterkillers, Buzzers, etc. Eighteen years later, Eggman was ready to enslave the world using his robotic army.
During the events of SMSS4, Metallicus is found in Mitchell's old lab by Bowser, who then uses him as a spy during the Interuniversal Civil War. But soon, Metallicus leaves his new master and goes his own way, not joining any specific faction.