Legend and Myth- British

Legends of these half-human, half-fish humanoids have circulated for millennia, even as far back as 5,000 B.C.[1] It has been widely suggested or implied that manatees or dugongs could be behind the myth of the mermaid. An example supporting this theory would be that Christopher Columbus had logged that he had seen mermaids on his journey to the new world, but thought they would be more attractive. These large aquatic mammals are notable for the way in which they carry their young, cradled in their arms much as a human would carry a baby. It is possible that sailors seeing these unfamiliar beasts for the first time, would assume that they had in fact stumbled across some sort of humanoid species, and consequently spread their accounts of the sightings through their homelands on their return from voyages. It has even been posited that the traditional image of a mermaid with long flowing hair could be attributed to manatees breaking the ocean surface underneath patches of seaweed, and giving the unfamiliar observer the impression of having long hair. Sightings from first-hand witnesses generally describe mermaids who do not talk and have green or black hair.[1]

[edit] Ancient Near East
Tales of mermaids are nearly universal. The first known mermaid stories appeared in Assyria, ca. 1000 BC. Atargatis, the mother of Assyrian queen Semiramis, was a goddess who loved a mortal shepherd and in the process killed him. Ashamed, she jumped into a lake to take the form of a fish, but the waters would not conceal her divine nature. Thereafter, she took the form of a mermaid — human above the waist, fish below — though the earliest representations of Atargatis showed her as being a fish with a human head and legs, similar to the Babylonian Ea. The Greeks recognized Atargatis under the name Derketo, where she was often conflated with Aphrodite.
Prior to 546 BCE, the Milesian philosopher Anaximander proposed that mankind had sprung from an aquatic species of animal. He thought that humans, with their extended infancy, could not have survived early on. This idea does not appear to have survived Anaximander's death.
A popular Greek legend has Alexander the Great's sister, Thessalonike, turn into a mermaid after her death.[2] She lived, it was said, in the Aegean and when sailors would encounter her, she would ask them only one question: "Is Alexander the king alive?" (Greek: Ζει ο βασιλιάς Αλέξανδρος;), to which the correct answer would be "He lives and still rules" (Greek: Ζει και βασιλεύει). Any other answer would spur her into a rage, where she transformed into a Gorgon and meant doom for the ship and every sailor onboard.
Lucian of Samosata in Syria (2nd century CE) in De Dea Syria ("Concerning the Syrian Goddess") wrote of the Syrian temples he had visited:
"Among them - Now that is the traditional story among them concerning the temple. But other men swear that Semiramis of Babylonia, whose deeds are many in Asia, also founded this site, and not for Hera Atargatis but for her own Mother, whose name was Derketo"
"I saw the likeness of Derketo in Phoenicia, a strange marvel. It is woman for half its length, but the other half, from thighs to feet, stretched out in a fish's tail. But the image in the Holy City is entirely a woman, and the grounds for their account are not very clear. They consider fishes to be sacred, and they never eat them; and though they eat all other fowls, they do not eat the dove, for she is holy so they believe. And these things are done, they believe, because of Derketo and Semiramis, the first because Derketo has the shape of a fish, and the other because ultimately Semiramis turned into a dove. Well, I may grant that the temple was a work of Semiramis perhaps; but that it belongs to Derketo I do not believe in any way. For among the Egyptians, some people do not eat fish, and that is not done to honor Derketo."[3]