Common carp were introduced to Japan long before koi existed and were farmed as food, particularly in the mountainous Niigata region where paddy irrigation ponds provided winter protein. Colour mutants appeared among these stock fish, and from roughly the 1820s farmers began isolating and breeding them for pattern rather than eating them.
The hobby stayed local for a century. Koi were shown at a Tokyo exposition in 1914, which spread interest through Japan, and international export followed after the Second World War, helped by the availability of oxygenated plastic bags for shipping live fish. Most of the recognised varieties, distinguished by colour arrangement and scale type, were fixed in the twentieth century.
Longevity claims deserve care. Koi commonly live several decades in good conditions, but the famous scarlet koi Hanako, reported at 226 years from scale-ring readings, rests on a single 1966 account and a method not considered reliable for that purpose.