Ningyo (人魚, "human fish"), often translated as "mermaid," is a fish-like creature from Japanese folklore.
Anciently, Ningyo were described with a mouth like a monkey's, small teeth like a fish's, shining golden scales, and a quiet voice like a skylark or a flute.
Like the kappa, they have an intricate society under the water and are also experienced in the ways of healing and magic. To consume the flesh of a ningyo in any quantity is to become immortal and the blood of a ningyo is said to heal any wound. However, to take the flesh or blood of a ningyo without his or her willingness to give it would undoubtedly also grant consequences such as the enmity of the ningyo as a people. Catching a ningyo was believed to bring storms and misfortune, so fishermen who caught these creatures were said to throw them back into the sea. A ningyo washed onto the beach was an omen of war or calamity.
One of the most famous folk stories concerning ningyo is called Happyaku Bikuni, "eight-hundred year Buddhist priestess". The story tells how a fisherman who lived in Wakasa Province once caught an unusual fish. In all his years fishing, he had never seen anything like it, so he invited his friends over to sample its meat.
One of the guests, however, peeked into the kitchen, noticed that the head of this fish had a human face, and warned the others not to eat it. So when the fisherman finished cooking and offered his guests the ningyo's grilled flesh, they secretly wrapped it in paper and hid it on their persons so that it could be discarded on the way home.
But one man, drunk on sake, forgot to throw the strange fish away. This man had a little daughter, who demanded a present when her father arrived home, and he carelessly gave her the fish. Coming to his senses, the father tried to stop her from eating it, fearing she would be poisoned, but he was too late and she finished it all. But as nothing particularly bad seemed to happen to the girl afterwards, the man did not worry about it for long.
Years passed, and the girl grew up and was married. But after that she did not age any more; she kept the same youthful appearance while her husband grew old and died. After many years of perpetual youth and being widowed again and again, the woman became a nun and wandered through various countries. Finally she returned to her hometown in Wakasa, where she ended her life at an age of 800 years. Today she is called the Happyaku Bikuni or Yao Bikuni, the "Eight Hundred Nun", because that is the age she lived to.
- Fishmen (魚人), often incorrectly referred to as Mermen, are a race who appear throughout the entire anime/manga series of One Piece on a regular basis. They look like humans with fish features and are obviously inspired by the ningyo. Fishman is written like ningyo but with the characters switched. (人魚, Ningyo -> 魚人, Gyojin) Merfolk (人魚) appear in the series too. These are more peaceful of nature than the Fishmen and, like the mermaids and mermen of folklore, their upper half is that of a human while the lower half is that of a fish.
- The manga/anime series, Mermaid's Scar and Mermaid Forest by Rumiko Takahashi is based on the Happyaku Bikuni myth, in which the main characters become immortal by consuming the flesh of a mermaid.