Melusine

Currently it is pretty common to depict mermaids as being able to grow legs when needed. As far as I know, that mainly dates from Splash. Before that, mermaids did not change form.

But, if you go back far enough, you find some exceptions. One is Melusine. I first ran across this term in an article about Starbuck's logo which is a topless, two-tailed mermaid. The article claimed that this creature is called a melusine.

It turns out that when you look it up, Melusine refers to a specific character in a number of medieval myths. She appeared to be a normal human most of the time but changed into a mermaid (or a half-serpent) on Saturdays. She married a noble (possibly a king) on condition that he not disturb her on Saturdays. Of course, he does eventually and she left him (possibly turning into a dragon on her way out).

Wikipedia concludes by saying that Melusine is a holdover water sprite from pre-Christian legends.