As you know mermaids are apart of a lot of mythologies. They were known as the daughters of Neptune or Triton. There are also apart of a sailors mythology the "Siren", people would categories Mermaid and Siren in two different categories, but they are actually the same, except in sailor mythology they dragged sailor men to the bottom on the ocean, some say they were trying to bring them to their underwater homes, but the sailors died.
Another mythology is that mermaids are the guardians of the 7 seas. The protectors of the creatures. Then there is one mythology that is a little more gruesome mermaids ate people. Their songs beautiful to lure men and women into a trans and then ate them.