The earliest known mermaid myth is 3,000 years old and originated in Assyria. The story goes that the goddess Atargatis accidentally killed her human lover. She was so distraught that she flung herself into a lake to live as a fish as punishment. However, she was too beautiful, so her top half couldn't be hidden. She then ended up with the tail of a fish and the upper body of a human woman.
This type of creature has become the popular image of a mermaid. A beautiful woman with a fishtail is found in a multitude of stories: both in folklore and in modern movies, shows, and books. Sometimes this also includes mermen. These men have the upper body of a male human with the tail of a fish. They are often pictured as highly attractive and physically fit.
This half-human and half-fish creature is what many think of at the mention of a "mermaid". However, there have been many mermaid myths created around the world. They always contain at least some human body parts and live predominantly in water. Yet their physical features, magical powers, and level of danger varies.