Yaobikuni

Ningyo (人魚, "human fish", often translated as "mermaid") is a fish-like creature from Japanese folklore. Anciently, it was described with a monkey’s mouth with small teeth like a fish’s, shining golden scales, and a quiet voice like a skylark or a flute. Its flesh is pleasant-tasting, and anyone who eats it will attain remarkable longevity. However, 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 Yao Bikuni (八百比丘尼, "eight-hundred (years) Buddhist priestess") or Happyaku Bikuni. 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.

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Wikipedia