Koi are domestic carp, that have been bred to be optically pleasing. They are related to goldfish but grow bigger.
Koi are domestic carp, bred to be optically pleasing. The japanese word koi (鯉) actually stands for carp.
Koi are related to goldfish, but grow bigger in size. Koi that have grown over 50cm are not unusual. The colors of koi vary, most commonly are red, white or black, or a mixture of two or more colors with different patterns.
Koi are often considered as a status symbol by their owners, as a result they can sometimes become very expensive in auctions. This holds true for big specimen or fish that sport a specific pattern. (E.g. white koi with a red dot on the head which resembles the Japanese flag.)