S04E22: Bull Fight Dance

Lucy literally blackmails Ricky into getting her a part in one of his guest appearances on television. He does get her a role -- as a bull. When Lucy is displeased with the turn of events, she transforms the bull's image from that of a snarling beast to a mincing creature resembling Elsie, the Borden Cow. Look for the classic scene where Lucy upstages Ricky in this episode.