According to "Manual to Nishikigoi," a book by Dr. Takeo Kuroki, the word "Koi" was first used about 2,500 years ago in China.
Confucius' son, born in 533 B.C., was presented a fish by King Shoko of Ro. The fish were used as the main subject in Chinese artwork and carvings and some Chinese rulers kept carp in captivity for their viewing pleasure.
While there may have been natural mutations of carp which featured patches of color on them in China, the Japanese are generally recognized as the creators of Nishikigoi (Living Jewels).
The Japanese were the first to take the naturally occurring mutations and develop them further. Japanese rice farmers kept them as food fish but somewhere between the 1820s and 1830s, they began to breed some of the carp for aesthetic appeal.
The farmers kept the colorful carp as pets for themselves. As the farmers developed different color types of Koi, interest in Koi spread throughout the prefecture (similar to a state in the United States) and then throughout Japan. National interest for Koi in Japan increased tremendously when Emperor Hirohito was presented Koi for the Imperial Palace moat in 1914.
Most people involved in the hobby consider the Niigata prefecture in Japan as the birthplace from which the Nishikigoi sprang. More specifically, areas in and around Ojiya City in Niigata are regarded as the home of Nishikigoi. Today there are more than 100 different color types and sub-types of Koi.
Wild carp were called "Koi" in Japan, but the term was also used to describe colored carp. The name Nishikigoi was given to these "colored Koi carp" during World War-II. Today colored carp are simply called Koi and the term has evolved into the common name for them worldwide.
The term Nishikigoi is used as a formal name. Nishikigoi is used to describe them in written text or describing the fish formally to Japanese people who do not have working knowledge of the fish.
Many people in Japan recognize the term Nishikigoi but may not be familiar with the term Koi.