Like a mermaid, but with 2 tails
Appears in European Heraldry, and a French medieval tale.
A French Medieval tale- Melusine married Raymond
of Poitou, although she forbid him to see her in the bath
one day out of the week.
They had children, and some of them had strange features like
big teeth. One day he spied on her in the bath, and saw that she
was part serpent or fish. At this she fled, never to be
seen again, but when a member of Melusine's family dies, a
dragon is said to fly over, crying. There are different
versions of this story.