Butterfly Koi

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.

Nishikigoi Judging
Butterfly koi cannot be judged using the traditional criteria of used for koi judging. The standard criteria used in these events have evolved over many years, and they are specifically tailored to rate the characteristics of koi. According to an article in KOI USA magazine, the following characteristics are largely the basis for the unsuitability of butterfly koi in traditional competition:

Popularity
Butterfly koi are strongly disliked by many keepers of traditional koi who view the breed as inferior to koi. This polarization of traditional keepers may be the reason why some koi retailers do not sell butterfly koi, and why many of Japan’s famous and most prestigious breeders do not breed butterfly koi today. They are largely unpopular in Europe and Asia, but are popular in the United States where they are more readily available. The popularity of these fish in the United States has earned them the nickname American Koi.`

http://en.wikipedia.org/wiki/Butterfly_Koi