The word "koi" comes from Japanese. Koi fish are specially bred in Japan for their color. A prize example can be valued in the $100,000 range. Koi are a type of carp and have been cultivated and bred in Japan for possibly thousands of years.
If you have been to Japan you will notice they are a common feature in most traditional Japanese gardens. They most likely originated from Eastern Asia, Aral, Black and Caspian Seas. Earliest records of Koi have been found in China and have been widely spread in Japan.