Half human and half fish or half human and half bird folklore lives and breathes in religious texts, literature, film, and big fish tales shooting from the mouths of drunken sailors spreading their over embellished stories of their voyages of the seas.  Folklore involving merfolk is embedded in cultures throughout the world, landlocked to coastal regions.

Era or Oannes

Babylonian deity Era aka Oannes is the Fish-god that is represented on seals and sculptures that date back to 5,000 BCE.  This fish god was usually depicted to have a bearded head with a crown and a half upper half man with a scaly fish tale instead of legs. This is the first known depiction of a merman.

Atargatis

Atargatis is a Syrian origin goddess whose influence spread to Greece, Rome, and beyond. Atargatis is the first depiction of a  mermaid.  Over the hundreds of years of being worshiped she was referred to be the goddess of fertility, goddess of the earth and water, and the goddess of love.  She is believed to be the direct inspiration to the Greek love goddess Aphrodite.

Folklore

The lore of merfolk can be greatly diverse from culture to culture.  Some folklore portray the mermaids/mermen as benevolent beings who are responsible for prosperous harvests. The morning dew on plants was believed to be the results of mermaids or water sprites dancing on land under the moon lit nights.  Other cultures perceive these merfolk to be malevolent beings that lure unsuspecting travelers with false promises of romance or luck that lead the victims to their watery deaths.

Rusalka…

a slavic myth of a ghost, water spirit, succubus or mermaid like demon that dwells at the bottom of the rivers, lakes, or wells.  Rusalki (plural) are spirits of young women who died a tragic death anywhere near a body of water.   In some versions, unbaptized babies who were drowned by their mothers were believed to be the creations of Rusalki.  Rusalki were cursed to live in the form of a mermaid and reside in the waters to where they originally met their fate.  They would sing enchanting melodies to entrap men, women, or children to their watery deaths.  Rusalki can live on water or land and are commonly described to be pale or to have translucent skin, and to have no visible pupils.  Some stories state that they have green fiery eyes with green or golden hair which is always wet.  This variation with the wet hair description states that if the hair of the Rusalka drys, she will die.  In some versions, the Rusalki had a positive effect on crops.

Sirens…

a Greek mythological creature described to be a half bird and half woman who lure sailors to their deaths with their beautiful melodic voices.  Homer, the ancient Greek poet, mostly known for The Iliad and The Odyssey, claimed to have seen two sirens on an island in the western sea between Aeaea and the rocks of Scylla.  In The Odyssey, the Greek Hero Odysseus wanted to hear the beautiful sounds of these creatures.  Under advisement from a sorceress named Circe, the crew stuffed wax into their ears to silence out the temptresses.  Odysseus had himself tied to the mast of the ship to prevent the temptation of the beautiful sounds that would lead to impending death.

A little History tidbit..

In 1492, Christopher Columbus sailed out from Spain to find a western trade route to Asia. Taken from his journal entries, On January 9, 1493, near the Dominican Republic, Columbus noted that he spotted three mermaids.  Later, the conclusion was drawn that Columbus could have mistaken manatees for the creatures that he described in his journal entry.

References

Sullivan, K. (n.d.). Rusalka: The Mythical Slavic Mermaid. Retrieved October 06, 2016, from http://www.ancient-origins.net/myths-legends/rusalka-mythical-slavic-mermaid-006738

Columbus mistakes manatees for mermaids. (n.d.). Retrieved October 06, 2016, from http://www.history.com/this-day-in-history/columbus-mistakes-manatees-for-mermaids

Took, T. (n.d.). Atargatis, the Phoenician Great Goddess. Retrieved October 06, 2016, from http://www.thaliatook.com/OGOD/atargatis.php

The Beautiful Monster: Mermaids. (n.d.). Retrieved October 06, 2016, from http://blog.biodiversitylibrary.org/2014/10/the-beautiful-monster-mermaids.html

Siren. (n.d.). Retrieved October 06, 2016, from https://www.britannica.com/topic/Siren-Greek-mythology

SEIRENES. (n.d.). Retrieved October 06, 2016, from http://www.theoi.com/Pontios/Seirenes.html

Sirens. (n.d.). Retrieved October 06, 2016, from http://www.greekmythology.com/Myths/Creatures/Sirens/sirens.html

Mermaids & Mermen: Facts & Legends. (n.d.). Retrieved October 06, 2016, from http://www.livescience.com/39882-mermaid.html