Butterfly koi, longfin koi, or dragon carp are a type of ornamental fish notable for their elongated finnage. The fish are a breed of the common carp, Cyprinus carpio, which includes numerous wild carp races as well as domesticated koi (nishikigoi).
In July 1977, the then Crown Prince Akihito visited the Saitama Prefectural Fisheries Experiment Station and suggested that there was a carp with a long fin in Indonesia (Cypirnus carpio var. flavipinnis C.V), and that it should be crossed with the Japanese pond smelt. This prompted the Station to start breeding koi in 1980, and two years later, this breed was born. They are still kept in a pond in the East Gardens of the Imperial Palace. These were known in Japan as onagagoi or hire naga nishikigoi, or translated in English "long tail carp". Randy LeFever, the son of Wyatt LeFever, a noted breeder of koi, is credited with suggesting they looked like butterflies, for which the breed is named. They are also sometimes referred to as dragon koi.