Bereginya
Bereginyas or Berehynias are obscure fairies mentioned in "The Lay of St. Gregory the Theologian of the Idols", which has been preserved in a 15th-century Novgorod manuscript. "The Lay" is a compilation of translations from Greek sources studded with comments by a 12th-century Kievan monk. The text, which seems to have been considerably revised by later scribes, does mention "vampires and bereginyas" as the earliest creatures worshipped by the Slavs, even before the cult of Perun was introduced in their lands. No detail about "bereginyas" are given, affording a large field for speculations of every kind.
Boris Rybakov connects the term with the Slavic word for "riverbank" and reasons that the term referred to Slavic mermaids, although, unlike rusalkas, they were benevolent in nature. The scholar identifies the worship of vampires and bereginyas as a form of "dualistic animism" practiced by the Slavs in the most ancient period of their history. According to him, the term was replaced by "rusalka" in most areas, surviving into the 20th century only in the Russian North. After the publication of Rybakov's research, the "bereginya" has become a popular concept with Slavic neo-pagans who conceive of it as a powerful pagan goddess rather than a mere water sprite.
Read more about this topic: Supernatural Beings In Slavic Folklore