Mermaids are usually depicted as beautiful creatures that have human form from the waist up, and the tail of a fish. There are conflicting accounts as to their character depending on where in the world you ask; in most places they are dangerous sirens, luring men (usually sailors and fishermen) to their deaths, although other stories exist of mermaids as omens of good fortune or kind and friendly creatures - notably on the Isle of Man in the United Kingdom.
The earliest known story of a mermaid dates back to Assyria around 1000BC, and tells of the goddess Atargatis who loved a mortal man but accidentally killed him. In her shame, she leaped into a lake intending to take the shape of a fish. But the waters would not hide her divine beauty, so she was only transformed below the waist, thus becoming what we know as a mermaid.
Another famous mermaid was Thessalonike, the sister of Alexander the Great,who was said to become a mermaid after her death. She roamed the Aegean sea forevermore, asking sailors she encountered whether her brother was alive. The correct answer was “He lives and reigns and conquers the world.” On hearing this answer she would calm the seas. If the answer came differently, she would condemn the ship, calling up great storms and dooming all onboard.