A ceasg is a mermaid in Scottish mythology, a supernatural half-woman and half-grilse (salmon). It is also known in Scottish Gaelic as maighdean na tuinne ("maid of the wave") or maighdean mhara ("maid of the sea"). The ceasg is said to be able to grant three wishes to anyone that captures her.
References
Ceasg Wikipedia(Text) CC BY-SA