If you look hard enough, 
You can find legends about mermaids all over the world,
The first known story about mermaids appeared in Assyria in 1000 BC.
A beautiful goddess fell in love with a shepherd, and accidentally killed him, a shamed, the goddess jumped into the lake in order to become a fish,
But the water could not compete with her beauty, so she became half fish half woman.
And this is how the first mermaid was created (if we believe the Assyrians)
But it is important to note that most of the stories describe mermaids as a kind of sirens who lure sailors to their deaths with the help of a song.