Mermaids (人魚 ningyo, lit. "human-fish") are traditionally more monstrous in Japanese folklore than most sources, having the upper bodies of monkeys rather than humans, or even just a human face on a fish body. A beached mermaid is an omen of disaster, while eating the flesh of a mermaid is said to grant long life or immortality.
Mermaids in Touhou
The mermaid race in Gensokyo appear to have been shaped by both native and foreign concepts, with the only known example having a human upper body and fish-fin ears. It is unknown whether monstrous mermaids also exist.