A mermaid is an aquatic creature with the head and upper body of a female human and the tail of a fish. Mermaids appear in the folklore of many cultures worldwide, including the Near East, Europe, Asia, and Africa. In ancient Assyria, the goddess Atargatis transformed herself into a mermaid out of shame for accidentally killing her human lover. Mermaids are sometimes associated with perilous events such as floods, storms, shipwrecks, and drownings. In other folk traditions (or sometimes within the same tradition), they can be benevolent or beneficent, bestowing boons or falling in love with humans. The male equivalent of the mermaid is the merman, also a familiar figure in folklore and heraldry. Although traditions about and sightings of mermen are less common than those of mermaids, they are generally assumed to co-exist with their female counterparts. The male and the female collectively are sometimes referred to as merfolk or merpeople.
Mermaids are expert swimmers, capable of swimming as fast or faster than a fish. Mermaids have the power to change into a human when dry. They can remain in human form until their legs are exposed to water or till low tide which causes their legs to involuntarily change back into a fish tail.
Mermaids are weak against fire and take twice the time to heal from burns than other injuries. They losing all abilities associated with being a mermaid. Essentially making them human. When in their human form they are prone to dehydration more easily than most species.
The Ancient Babylonians had a mergod called Era who was depicted as half man, half fish. He was the god of the sea (handy that being part fish and all) the Syrians decided to copy the Babylonians and created their own merged, Derketo. Derketo was depicted in the more traditional way of top half of a lady and bottom half of a fish. This idea was in turn nicked by the Ancient Greeks, who came up with the idea of Sirens. Siren’s in Greek mythology often get cited as a starting point for the more recognisable mermaid mythology. Half female, half bird creatures, they morphed into sexy fish fancies, luring sailors to their death on the rocks by singing sweet songs and flashing their knockers.