Why a Duck? is a comedy routine which features in the Marx Brothers movie The Cocoanuts. In a scene in which Groucho and Chico are discussing a map, Groucho mentions the presence of a viaduct next to a peninsula, to which Chico, who is playing the role of an immigrant with poor English skills, replies "Why a duck?" This leads into a long schtick positing the existence of "Why a chickens?", "Why a horses?", etc.
Why a Duck? is a touchstone scene for Marx Brothers fans, as evidenced by a book of the same name (featuring a foreword by Groucho) and a fansite, both of which focus on the minutiae of the Marx Brothers' routines.