Mermaids have been called many things such as "Merfolk." They have been mistaken for water nymphs because they are supposed to have the beautiful voices liker water nymphs. They have been mistaken for Sirens as well except Sirens have the upper body of a young woman but the lower body of a bird.
The Babilonians worshipped a sea-god, the mermaid, and believed she arose form the sea to teach man the arts and sciences but were under impression that the upper body was man.
The Syrians and Philistines worshipped a mermaid moon goddess and was shown as a mermaid as the tides ebbed and flowed with the moon.
In Japanese and Chinese legends were not just mermaids. They believed them to be sea-dragons and the dragon-wives. The Polynesian myths showed them as half human and half porpoise.
Greek and Roman myths are very similar. These cultures have some of the first descriptions of the mermaid and the mermen.
Oxid wrote that the mermaids were born from the burning galleys of the Trojans where the timbers turned into flesh and blessed and the "green daughters of the sea."
Poseidon and Neptune were depicted as half-man and half-fish and were mermen.
The British call them Merrymaids the Irish call them Merrews. They believed they lived on dry land below the sea and had enchanted caps that allowed them to pass through the water without drowning.
Some believe they can only live in fresh-water while others believe only salt-water. The myths are endless.