Koi is an informal group of the colored variants of C. carpio. Several varieties are recognized by the Japanese. Koi varieties are distinguished by coloration, patterning, and scalation. Butterfly koi (also known as longfin koi, or dragon carp), also developed in the 1980s, are notable for their long and flowing fins.

Some of the major colors are white, black, red, orange, yellow, blue, and cream. The most popular category of koi is the Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.