 The Butterfly Koi or, Long-fin Koi, is reported to be hardier than the standard fin. This is based on genetics with what is called hybrid vigor (heterosis). The Butterfly and Standard fin will inter-breed easily. The Butterfly has beautiful pectoral, ventral, dorsal, and caudal fins that are long, flowing and almost featherlike in appearance.
Butterfly Koi are truly a beautiful Koi because of their fins. They are available in exactly the same colors, scale patterns, and metallic as standard fin Koi, and have the same names.
Butterfly Koi are what the Chinese describe as "swimming flowers", and the Japanese as "living jewels".
The Butterfly Koi breeding program originated in the United States in the early 1980's. At that time, Wyatt Lefever obtained some unusual long-finned hybrid Asian carp. He started a breeding program crossing them with metallic colored Koi resulting in "Butterfly Koi".
Butterfly (long-finned) Koi are not only beautiful with long flowing fins, but they are quite hardy and able to withstand colder temperatures better than many traditional Koi.
Butterfly Koi have been available in the United States for seven or eight years.
Butterfly Koi are not a hybrid of goldfish or Koi as many people believe. Koi and goldfish being closely related will readily interbreed but the resulting progeny are unattractive, mostly resembling wild carp with very little color.
Butterfly Koi start to develop elongated fins and tails at about six months of age, but really start coming into their own at about a year. After a year or two, their fins and tail will be almost as long as their bodies. Butterfly Koi need to grow rather slowly, lest their bodies outgrow their fins and ruin the butterfly effect. | 
