In Slavic paganism there are a variety of female tutelary spirits associated with water. They have been compared to the Greek Nymphs, and they may be either white (beneficent) or black (maleficent). They may be called Navki, Rusalki, and Vily. The Proto-Slavic root *navь-, which forms one of the names for these beings, means "dead", as these minor goddesses are conceived as the spirits of dead children or young women. They are represented as half-naked beautiful girls with long hair, but in the South Slavic tradition also as birds who soar in the depths of the skies.
https://en.wikipedia.org/wiki/Slavic_water_spirits