Ningyo as the name suggests, is a creature with both human and fish-like features, described in various pieces of Japanese literature.
Japanese mermaid (ningyo). Coiffed with nihongami hairstyle of the Edo Period.― Santō Kyōden Hakoiri musume menya ningyō (1791)
The ningyo (人魚) aka ryōgyo (鯪魚, 'hill-fish') (cf. full image)
The Teijin (氐人), or the "Di people"
A ningyo (人魚), aka Teijin (氐人) or a Di countryman―Toriyama Sekien's Konjaku Hyakki Shūi (1781)
A merman, the male counterpart of the mythical female mermaid, is a legendary creature which is human from the waist up and fish-like from the waist down, but may assume normal human shape. Sometimes mermen are described as hideous and other times as handsome.
Merman
Triton with a nymph
Triton ―Schott's Physica Curiosa (1697).
The merman (1911) by John Bauer