It was explained that Armon lost his real arm fighting in the Pharaoh's army back when he was still alive. The Pharaoh saw this, and knew that he would be the perfect bodyguard for his son. The pharaoh then gave him his golden arm.
Also, Armon taught the prince the fighting ways of his people, including the art of using weapons, Egyp-Tsu.
While not fighting, Armon spends most of his time eating or watching 'the magic box' TV that Presley gave them.
Out of all the mummies, Armon is the most understanding. He's always there for Presley and the others, ready to help whenever things get rough.