Now here’s a really popular romance of the Middle Ages: Melusine of Lusignan, as told by Jean d’Arras and company.
Melusine is normally drawn with a fish tail or tails. (In fact, the German woodcut that Starbucks first used for their trademark was not a mermaid but a melusine, as you can tell by the lack of mirror and the twin tails.) But in the actual legend, she had a lower half that was a snake (or dragon!) on Saturdays.