This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Ceasg (Scottish)
The ceasg is a mermaid in Scottish folklore with the upper body of a beautiful woman merging with the tail of a young salmon. She is also known in Scottish Gaelic as maighdean na tuinne ("maid of the wave") or maighdean mhara ("maid of the sea"). The ceasg lives not only in the sea but also in rivers and streams, and can be made to grant three wishes to anyone who captures her. Marriages sometimes occur between ceasg and humans, and famous maritime pilots are often reputed to be descended from such unions.
Even when these marriages end and the ceasg returns to the sea, they will always take an interest in their human descendants, protecting them in storms or guiding them to the best fishing grounds. The ceasg is sometimes imagined as something more monstrous. In some tales she swallows the hero and he remains alive in her stomach. The hero's wife plays a harp until the mermaid is charmed and the hero escapes.