Le Ban - Mermaid - Saint: The Irish Mythical Tales, #2

Lí Ban — The Irish Mermaid-Saint
A woman transformed into a half-salmon mermaid after a flood destroys her home. She lives underwater for a year with her dog (also transformed), then emerges centuries later to be baptised by Christian monks. Why it's unused: A mermaid caught between pagan magic and early Christianity offers rich thematic conflict.