Petrov’s “Mermaid” seems based on Dargomyzhsky’s opera “Rusalka” (”drowned maiden”) which is loosely based on Pushkin’s incomplete poem of the same name.

In the opera, and in the animation, the Rusalka is a woman who, having given herself to a prince, is jilted as he marries another woman. She is present at the wedding. Pushkins poem is more about the seduction and death of the hermit, and doesn’t bother with this history.

Traditionally, Rusalka are sort of undead: woman who die before their time; women who commit suicide because they are jilted by their lovers; unmarried women who are pregenant. They are allowed to prperly die when their death is avenged. There are many other traditional characteristics of the rusalka in Petrov’s film.
(Many thanks to online encylopedia’s)