Melusine
Melusine is a fresh-water spirit of French folklore, often a woman who becomes a serpent or mermaid below the waist, sometimes winged. In the medieval romance she married Raymondin of Lusignan, provided he never saw her on Saturdays; when he broke the oath she fled as a serpent. She is the legendary ancestress of the Lusignan, Anjou, and Luxembourg dynasties.