Why is Robert the Bruce called Robert "the" Bruce?

by SeeBoar

Why is "the" added into his name? Not like there was William the Wallace

bfg_foo

Robert's family name was derived from "de Brus," which means "of Brus/Bruce." However, in the case of his family, it was sometimes anglicized to "the Bruce" instead. He is sometimes referred to as simply "Robert Bruce."