Nobody really knows how Cerberus came into being. He is the only one of his kind and seems to be incredibly old. Stories go back as far as the foundation of Toyris about him having the exact same size and appearance that he does now.
Cerebus “holds” the entrance of the Fallen Valley northeast of Raikfort where he lives primarily off goats, bison, and the odd offering from either Aides or the Oread Nymphs. The Nymphs appreciate the effect he has on Satyr, Human, and Protomen- scaring them away from the valley.