Mr. Slate is Fred's hot-tempered boss at the stone quarry. Though he is friends with Fred and Barney, he is often not impressed with Fred's antics at the quarry, and has fired him on many occasions, only to give him his job back at the end. A running gag is his ever-changing first name, which has been presented as Sylvester, Nate, Oscar and George.
Note: The more recently recognized Mr. Slate at first was called "Mr. Rockhead", and Mr. Slate was a shorter character with black hair. Without any explanation, the two characters switched identities, and the now short "Mr. Rockhead" slowly faded out of existence.