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").
Butterfly Koi originated in the mid-20th century as a result of an effort to increase the hardiness of traditional koi. Japanese breeders interbred wild Indonesian Longfin river carp with traditional koi. The resulting fish had longer fins, long barbells, pompom nostrils, and were hardier than koi. These were known in Japan as "onagaoi" or "hire naga koi", or translated in English "long tail koi". Randy LeFever, the son of Wyatt LeFever, a noted breeder of koi, is credited with suggesting they looked like butterflies, a trait for which the breed is named. They are also sometimes referred to as Dragon Koi. They do not get as big or live as long as normal Koi.