They are believed to have originated from Eastern China. Earliest records of Koi have been found in China and have been widely spread in Korea and Japan. The ornamental cultivation of carp flourished under the Chinese Qing Dynasty, Korean Silla Dynasty, and Japanese Edo Period. They are very closely related to goldfish and, in fact, the of breeding and ornamentation has become very similar, probably through the efforts of Japanese breeders to emulate goldfish.
The word "koi" comes from the Japanese, simply meaning "carp." It includes both the dull grey fish and the brightly colored varieties. A homophone of koi means "love, affection" and koi are therefore symbols of love and friendship in Japan: a good example is the short story Koi-san by Mukoda Kuniko.
Photo credits: Forshone.