The history and mythology of koi goes back to China, over 2,000 years ago, and these images and stories were absorbed into Japanese culture like so many other things. They have been kept as auspicious pets dating back hundreds of years. They have been popular images throughout the history of Japanese tattoo designs and as that style is now very prevalent in Western tattoo culture, so is the Japanese koi tattoo.
In real-life, these fish are ornamental carp kept in backyard ponds and water gardens. They were bred from ordinary fish to develop their distinct and fantastic patterns and colors. The only thing more fantastical than the array of colors that these fish have in the real world is the patterns and colors they are depicted with in tattoo designs.
Koi is the Japanese word for "brocaded carp" and refers to the large, colorful fish that have been bred and kept in ponds for centuries. The formal name Nishigikoi translates as "living jewels." Careful breeding of these fish really got a modern foothold in the 1800s, and it has produced a rainbow of colors and a variety of specific body markings in the years since.
They are part of Japanese mythology and are symbols of strength, perseverance and courage. These fish are very long-lived under the right conditions and this is what has contributed to these interpretations and associations. In myth, the very strong koi will be able to swim upstream and leap the waterfalls, being rewarded at the end of the journey by being turned into a dragon.