mermaid; merman
Refers to the mythical creature with the upper body of a human and the tail of a fish. Can be used for both male and female, though often associated with female mermaids.
There is a story of the Little Mermaid in Andersen's fairy tales.
その絵本には美しい人魚が描かれていた。
A beautiful mermaid was drawn in that picture book.
Used for animals and fantastical creature-like beings; in practice this is the natural choice for 人魚.
A Sino-Japanese borrowing of Chinese 人魚, literally “human-fish,” used for legendary beings combining human and fishlike features.