I think Koi are the most beautiful variant of Carp there is.
They're so cool looking, that's why I love Koi ponds so much too.
I actually have a butterfly Koi above my bed, it's sort of like an elongated koi, they're often called Dragon Carp
That's really awesome, how big of a tank do you need for them?
For a Koi they generally need a large body of water and function best in coldwater, mine came directly from their home in Japan haha.
Must be why Koi ponds are a thing, since tanks are probably not the most ideal place for them to thrive.
Koi pond
Koi ponds are ponds used for holding koi, usually as part of a landscape. Koi ponds can be designed specifically to promote health and growth of the Nishikigoi or Japanese Ornamental Carp. 
The architecture of the koi pond can have a great effect on the health and well being of the koi. The practice of keeping koi often revolves around "finishing" a koi at the right time. The concept of finishing means that the fish has reached its highest potential. Koi clubs hold shows where koi keepers bring
Butterfly Koi
Butterfly koi, longfin koi, or dragon carp are a type of ornamental fish notable for their elongated finnage. The fish are a breed of the common carp, "Cyprinus carpio", which includes numerous wild carp races as well as domesticated koi ("Nishikigoi").
Butterfly koi originated in the mid-20th century as a result of an effort to increase the hardiness of traditional koi. Japanese breeders interbred wild Indonesian longfin river carp with traditional koi. The resulting fish had longer fins, long barbells,