Fifth Beatle

someone who missed out on success. The original "Fifth Beatle" was Brian Epstein (1934-67), the group's manager, so dubbed, much to his annoyance, by Murray the K, an American disc jockey, in 1964. Others could more fittingly have merited the title - Stu Sutcliffe, an early member of the group who was eased out and died before fame struck. Pete Best, who was replaced as the drummer by Ringo Starr, and Neil Aspinall, road manager, aide and friend.

Source Word and Phrase Origins, Nigel Rees