In folklore, a mermaid is an aquatic creature with the head and upper body of a female human and the tail of a fish. Mermaids appear in the folklore of many cultures worldwide, including Europe, Asia, and Africa.
Mermaid
The Fisherman and the Syren, by Frederic Leighton, c. 1856–1858
Siren and onocentaur.―Bern Physiologus. Berner Burgerbibliothek, Cod. 281, fol. 13v
Siren in a Second Family bestiary―British Library MS Add. 11283, fol. 20v.
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