Ceasg

The ceasg (also maighdean na tuinne, 'maid of the wave,' or maighdean mhara, 'maid of the sea') is a Scottish Gaelic mermaid distinguished from the standard European mermaid by having the lower body of a grilse—a young salmon—rather than a generic fish tail. This salmon-body marks her out as a freshwater as well as a sea creature; ceasgan are recorded in lochs and rivers as well as the open sea, reflecting the salmon's own migration between salt and fresh water. The Scottish folklorist Donald MacKenzie suggested that the ceasg may originally have been a sea goddess to whom offerings—possibly human—were made, and her transformation into a wish-granting captive represents the domestication of an older and more fearful tradition.

If a man can capture a ceasg, she is obliged to grant him three wishes in return for her release. In some variants she sheds her fish tail permanently to assume human form, marrying into the human world and bearing children who become famed sailors and pilots—inheriting their mother's mastery of water. However, the marriages are uniformly temporary: something always reveals her true nature and she returns to the sea, leaving her human family behind. The Kessock tradition—the best-documented local variant—differs in that the transformation is forced by removing scales from her tail; this variant emphasises violation and captivity rather than a bargain freely made.

The ceasg's song is described as irresistible, luring men into the water to drown. This double nature—a wish-granting wife on one hand, a lethal siren on the other—places her in the broader tradition of ambivalent water-women that includes the Welsh Gwragedd Annwn (lake maidens of the Otherworld) and the Irish merrow. The Spooky Isles article on the ceasg and the Scottish folklore survey at Culloden Battlefield both note that descendants of men who married ceasgan were said to carry a trace of supernatural luck at sea—a benevolent inheritance from their vanished mothers.