BOSTON -- William Watts Biggers, co-creator of the cartoon Underdog, the mild-mannered canine shoeshine boy who turned into a caped superhero to rescue his girlfriend, Sweet Polly Purebred, has died. He was 85.Family friend Derek Tague said Mr. Biggers, who went by "Buck," died unexpectedly at his Plymouth, Mass., home Sunday.The native of Avondale Estates, Ga., worked for the New York City advertising firm DFS and was asked by the agency's largest client, General Mills, to create television cartoons to promote its breakfast cereals. The most famous was Underdog, which debuted on NBC in 1964.The canine superhero, voiced by comic actor Wally Cox, also battled villains, including mad scientist Simon Bar Sinister and a gangster wolf, Riff Raff.Upon hearing the cries of Sweet Polly Purebred, Underdog rushed into a telephone booth to transform into the hero.Among his most famous lines: "There's no need to fear, Underdog is here."After General Mills pulled out of the animation business, Mr. Biggers became vice president of promotion and creative services at NBC.He also wrote for publications, including TV Guide, Family Circle and Reader's Digest, and wrote several novels, including The Man Inside and Hold Back the Tide. He is survived by a daughter, Victoria Biggers; a son, W. Watts Biggers Jr.; and longtime companion Nancy Purbeck.