mermaid, in folklore, sea-dwelling creature commonly represented as having the head and body of a woman and a fishtail instead of legs. Belief in mermaids, and in their counterpart, mermen, has existed since earliest times. They are often described as having great beauty and charm, which they use to lure sailors to their deaths (see Siren). In some legends they assumed human shape and married mortals (see Mélusine). The origin of the mermaid is thought by some to be the dugong (see sirenian).