Koi , or more specifically nishikigoi , are colored varieties of carp that are kept for decorative purposes in outdoor koi ponds or water gardens.
Koi
Terraced rice paddies in Yamakoshi, Niigata Prefecture
Illustration of a three-colored carp in Ritsurin Garden, 1900. This is the oldest illustration of koi. It has the kanji characters for asagi on its back and red on its belly.
Odd-eyed cat and Spotted carp, from the magazine "Shonen.", 1910.
Cyprinus is the genus of typical carps in family Cyprinidae. Most species in the genus are of East Asia origin with only the common carp in Western Asia and Europe; this invasive species has also been introduced to many other regions around the world. Cyprinus are closely related to some more barb-like genera, such as Cyclocheilichthys and Barbonymus (tinfoils). The crucian carps (Carassius) of western Eurasia, which include the goldfish, are apparently not as closely related.
Cyprinus
Japanese koi (ornamental domesticated carp) with various colors