There are few tales of the fairy folks' friends that are as captivating as those that deal with the mermaid, those ocean-dwelling divinities that are half-human and half-fish. Although there are mermen, the greater fascination has always been on the mermaid with her top half a beautiful woman and her bottom half that of a fish. Traditionally, the mermaid is also gifted with a lovely singing voice, which can be used to warn sailors of approaching storms or jagged rocks ahead. Or, in many of the ancient stories, the seductive siren song of the mermaids lure the seamen onto the jagged rocks and to their deaths. As with all of the "middle-folk," mermaids can be agents of good or of destruction.
As in the folklore of the selkie, sometimes mermaids fall in love with humans and are able to come ashore in human shape and to live on land for many years. They may even have children with their human husbands. But in all of these tales of mercreatures and human mates, the mermaid longs to return to the sea, and one day she will leave her human family and do so.
The Ceasg is a type of mermaid that haunts the lakes of the Scottish highlands. Her upper body and facial features are those of a beautiful, well-endowed woman, but her lower half is that of a large salmon. Like most supernatural beings, the Ceasg is of a dual nature. If a handsome young man should capture her attention and treat her well, she may assume human shape and marry him, at the same time granting him three wishes that may make them wealthy. If she feels that she has been disrespected or treated badly, she may use her beauty to lure a fisherman to the deepest part of the lake and drown him.
In Lake Tanganyika in the small East African country of Burundi, the Mambu-mutu is very much the classical mermaid, half attractive woman and half large fish. In her case, however, she does not fancy humans, and her only intention is to drag them under the lake's surface and suck their blood.
In Estonian folklore, the Nakh are shape-shifting water demons who walk freely on land as handsome young men or beautiful women who lure their victims with the sound of their sweet, seductive singing. Once they have enchanted their victims, they lead them to river, lake, or ocean and entice them to watery graves.
The Nix is a particularly nasty shape-shifting entity who, like all the fairy folk, loves to dance. According to German folklore, the Nix are attracted to the sound of music at fairs, carnivals, or outdoor concerts, and they appear as attractive men or women who enthrall the human audience with their skill and grace on the dance floor. Once they have lured a charmed human to join them at water's edge with the promise of romantic dalliance, they reveal themselves to be ugly, green-skinned fairies who drag their victims into the water and death by drowning.