Koi were created by man. They do not exist in the wild any more than the white fluffy sheep we have on farms exist in the wild. They do have a wild counterpart - the black common carp (Cyprinus carpio) - found throughout Europe. It is generally accepted that these common carp were taken from Europe to China, Korea, then Japan - primarily as a source of food. That process started about 2,000 years ago. Slowly, they were traded a little further east, then a little further, and still further, taking about 1,000 years before they reached Japan.
The farmers of Asia were pretty smart. By keeping these common carp in their rice paddies, they developed a secondary high protein food source. Being confined to their rice paddy ponds, these carp were terribly inbred. Mutations in colors developed. Just as people kept the friendlier wolf pups, who were the ancestors of our pet dogs as pets, these farmers kept the carp that came in unusual colors as pets because they were a cool thing to have.
These different color carp acquired the name "nishikigoi," which literally means "brocaded carp" in Japanese. Koi is short for nishikigoi. The word "koi' was first known to be used is a story about Confucius in 500 B.C. Evidently King Shoko, from a place called Ro, gave Confucius a koi as a gift when Confucius' first son was born because carp were considered a symbol of strength.
Koi are now bred all over the world. There are thriving koi breeding industries in the United States, Southeast Asia, and Israel. Those koi, which are also quite beautiful, are usually much less expensive to purchase because they just don't have the panache of being a true nishikigoi from Japan. It's a snob appeal thing.
Koi as we know them are not an ancient fish. Yes there are carp in very old Chinese and Japanese paintings, but these were of black or gray fish. It was only about 200 years ago in Niigata perfecture (sort of a county) in Japan that koi cross-breeding to create the fish we know today began. It was around 1840 that the first "kohaku" - that's a red and white koi - was first produced. Today there are hundreds, and hundreds of varieties. Actually no two koi look exactly alike. The best koi are still being produced in Niigata perfecture where there are about 600 small family koi "farms" breeding these extraordinary fish.
Since they are pond fish, koi are "designed" to be looked at from above, as people generally look down into a pond. So what determines a koi's quality is how they are shaped and patterned as you look down their backs (or dorsal fin). In an aquarium shop there may be a few small koi in a glass tank. Koi specialty facilities have many ponds, always allowing you to look down at where the fish are swimming so you can better judge the way it looks.
From a newly hatched "fry" - baby fish - about 1/16th of an inch long, koi can grow up to 40 inches in length. That's about the size of a big salmon! Koi are not aggressive, although there is a lot of pushing and shoving when they feed. You'd swear they were all starving. Koi don't have an "off" switch - they just eat, and eat, and eat. Since they don't have a stomach - just a long gut - if they consume more than they can hold, it just comes right out the other end. They don't bite because they can't. Their "teeth" are located sort of where our jaws hinge. They suck food into their mouths, decide whether its something they'd like to eat before they'll chew it, otherwise they just spit it out. A distinctive feature on koi are a set (one large and one small)of "barbels" - on each side of their mouth. They look like whiskers, but they're actually little feelers - not unlike catfish. Koi have big downward pointing mouths because they are bottom feeders, and that shape lets them root through stuff at the bottom of a pond.
While its hard to think of a fish as having personalities, koi do. They're very curious and get into everything. They love to dig up pond plants looking for more to eat. Koi don't like having their world changed and will have a hissy fit and not come up to play for days if they're moved. Sometimes they'll swim to the bottom with their faces against the side of the pond if they're not comfortable with a visitor. Their eye sight is good up to about 10 feet. Koi can visually recognize their care givers. They are also extremely sensitive to vibrations. Their primary connection to the world is via sonar. They can tell the difference between the sound of the footsteps of the person who feeds them and those of a stranger. Koi can hear, and they can taste. I have one who refuses to eat lettuce while her ponds mates love it - just like a small child.
When they are spooked, or when they don't like the water - they'll jump. Sometimes they'll jump right out of the pond. It takes them time to adjust to water that is different than what they're use to. During this period of adjustment they've been known to die because they've jumped out of the pond and there's no one around to put them back in. With new koi its always a good idea of have a net over the pond until they're settled. At koi shows you'll sometimes see a lot of "flashing" - sort of flipping around in the water, and jumping, because even though the water is of excellent quality, its different than what they're use to and they don't like it.
Conversely, koi jumping can mean that the water quality is so bad that the fish are desperately trying to get away from it in an attempt to find something better. Test you water and make sure there there is no ammonia, no nitrite, very little nitrate, ph around 7.5 to 8.0! Testing the water quality is easy - almost every aquarium shop has inexpensive and simple to use test kits for sale. Fish flashing can also mean something is irritating their skin. This is a strong indication that they may have parasites - check that out too!
Koi can live a long, long time. There is a story that one fish, who lived in a Japanese monastery, where I guess they can keep track of this sort of thing, who was over 200 years old when she died! Some of the koi that were exhibited at the 1914 Tokyo Exhibition are still swimming around the Imperial Palace today. Unfortunately, the average life span of a koi when it leaves the shop to its new home is 12 weeks. So learn about what type of care your koi requires - then they can be a part of your life, your children's lives, and your grandchildren's lives. Koi are truly "living jewels" - take good care of them.