As it is said that 'a typical person from Edo is the streamer of carp in May'; and although koi-nobori (鯉幟) is called 'nobori' (幟) (flag), it's a streamer whose shape mimics a fish.
「江戸っ子は皐月の鯉の吹流し」と言われる様に、鯉幟は「幟」とは名づけられているものの形状は魚を模した吹流し形である。