"Ningyo" is the name given to the traditional Japanese mermaid. They are generally helpful creatures that warn human of encroaching dangers and were common sights in the waters of Japan for a long time.
The appearance of the Japanese mermaid is generally vastly different from the ones typically portrayed in Western cultures. Exact details vary somewhat. Mostly they were described as a giant fish with a human face and a full head of hair. Sometimes they have horns, sometimes fangs. In a few descriptions the were half human and half fish, but the human half was grotesque, resembling a monkey more than a human and with scaly arms and clawed hands. Another depiction of mermaids portrayed as more human, but with distorted or demonic features, alabaster skin, and voices like flutes or songbirds.
The more familiar Western mermaid would come to influence how the Japanese viewed the ningyo as the stories and traditions made their way into Eastern culture.
The ningyo are known to appear in order to foretell things. Amabie was a mermaid that appeared to predict six years of good harvest, saying that if disease were to fall upon them that they should show a picture of her to the afflicted to cure them. Amahiko and Arie were both ningyo with similar stories. All three were fish from the waist down, but were covered in scales with lips like a dome-covered cylinder.
Ningyo are said to be able to shift shape and cry tears of pearls. Some specific traditions depict them using the shape shifting abilities to lure people into the ocean to kill them. The flesh of the mermaid had an ability that may have been heavily sought after; was believed to convey eternal youth and beauty on whomever ate it.
Translated as "eight-hundred year Buddhist Priestess", it is one of the more popular legends concerning ningyo.
According to the story, a fisherman once caught a very strange fish, unlike any he'd ever seen. He invited his friends over to try it, but one guest peeked into the kitchen and saw that the fish had a human face. He warned the others not to eat it, and so when the grilled fish was given to them, they hid the meat in scraps of paper.
One particular guest had a little daughter at home, who demanded a present when he returned from the gathering. He went home, and, drunk on sake, unthinkingly gave her the fish. By the time her father came to his senses she'd already eaten it all up. Although he'd worried that she may be poisoned, she showed no ill effects so he put it out of his mind.
She grew up and married, but while her husband grew old and died she stayed young. She was married and widowed many times before she decided to become a nun and wander the world. At her 800th year she returned to her hometown and ended her life.