Koi, Cyprinus carpio, in Japanese garden pond.
Koi (錦鯉; shortened from Japanese nishikigoi) come in an infinite variety of colors and patterns, distilled from the bland coloration of their wild carp forebears through centuries of selective breeding.
Koi varieties are distinguished by coloration, patterning, and scalation. Butterfly koi, developed in the 1980s and notable for their long and flowing fins, are actually hybrids with Asian carp, and not considered true koi.
Koi breeding is generally held to have begun around the 17th century in the Niigata prefecture of Japan. Farmers working the rice fields would notice that some carp would be more brightly colored than others, capture them, and raise them (when normally the brighter colors would doom the fish to be more likely eaten by birds and other predators). By the 19th century, a number of color patterns had been established, most notably the red-and-white kohaku, but the outside world did not become aware of the degree of development until 1914, when the Niigata koi were exhibited in Tokyo, and some of them presented to Crown Prince Hirohito. At that point, interest in koi exploded throughout Japan and later worldwide.