Mermaids are pretty versatile legends. Stories of mermaids have been told across Asia, Africa, and Europe for a long long time. Christopher Columbus was convinced that he spotted mermaids in the Caribbean. In fact - even within this century, there are reports of apparent mermaid sightings!
The first known mermaid story is from Assyria, c. 1000BC. It's about this goddess, Atargatis, who falls in love with a mortal shepherd and their love got him killed. She felt awful about it that she banished herself to the sea in the form of a fish. However, she could not conceal her beauty and so her body took shape as a mermaid.