Butterfly koi fish are koi fish varieties having long fins.
More detailed infor from https://www.koisale.com/butterfly-koi.html
Butterfly Koi, also called Long Fin Koi, are our passion. We truly love producing the best butterfly Koi in the world, hands down. Contrary to some writings , Butterfly Koi fish were firsts produced by our friends at Suda Fish Farms near Ojiya City Japan where Mr Suda crossed a Indonesian long fin carp with his parent Koi, not to produce long fin Koi or butterfly Koi, But to IMPROVE (Yes, improve!) the body.
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The result were some Koi that looked like good Showa, some Koi that were very low quality, And some that we're the first Butterfly Koi to be produced.
These first generation Butterfly Koi were raised and some were sold. Mr Suda continued to produce some of these butterfly Koi for sale but these fish were not readily accepted in the market.
Today butterfly Koi are extremely popular and found competing at Koi shows and residing in high end Koi ponds throughout the world. Mr Suda has since produced some very nice Butterfly Koi, especially in the Showa variety.