The main problem in any folkloristic subject, especially those reaching back to pre-Christian times is a complete lack of any form of canon. In addition, very limited connections between social groups on one hand and rather intense migrations on the other make the reconstruction of the specific beliefs of a specific group very hard if not downright impossible. General statements that the 'being X (say, rusalka) had such and such traits' is generally false by definition, because different societies, even residing in the same general area could have referred to such a being by a different name and attributed more or less different traits to them. Given that save for generally rare notes about folk beliefs made in the Middle Ages, most folklorist knowledge is based on the ethnologist works of late 18th and 19th century, certain beliefs could have been thoroughly transformed over the centuries, or sometimes even millennia and thus we can't be sure how they did look like at a specific point in time.

The mention of 19th century is most likely not a reference to actual folk beliefs, but rather to the representation of the aquatic supernatural beings in literature, that started to commonly utilize folklore in the Romantic period, sometimes precisely to popularize such folk (and thus indigenous) beliefs among the compatriots as a part of the budding modern national identity. Given that authors were recruiting chiefly from the urban upper or middle class, such depictions were most often than not a marriage of actual local folklore with the classical Roman and Greek myths most educated people were familiar with. In this case, a prototypal text would be Undine by Friedrich de la Motte Fouqué, published in 1811 that was quickly adopted into opera by E.T.A. Hoffman in 1816 (and by Albert Lortzing in 1845) and numerous other works, including H.C. Andersen's Little Mermaid (1937), Undina by Pyotr Tchaikovsky (1869) or Antonin Dvořák's Rusalka (1901), using both the German original and Andersen's interpretation as the source of inspiration. In case of rusalkas, even the name is pretty modern, as it is largely thought to be used only since 18th century and Russian rural dwellers generally considered the term foreign or 'bookish', unlike actually old names such as mavka (sometimes also navka), vodyanikha or beryeginya (from rus. 'byeryeg' or 'shore').

Idea of aquatic spirits spirits that are either benevolent or indifferent towards people and focused on their own domain, is present in the Greek mythology and exemplified there by nymphs (e.g. sea Nereids or freshwater Naiads). These beings were usually a form of the protective spirits embodying the nature of its place of residence and could have been helpful to people showing proper reverence or dangerous to those who attempted to defile her abode.

Some folklorists point out that the modern image of rusalka as a beautiful, but dangerous female spirit in Slavic folklore is a fusion between several types of aquatic beings commonly existing in folk beliefs of Central and Eastern Europe. One of them would be south Slavic wiła, a capricious spirit, usually closely tied to a specific body of water, that could have been helpful or harmful for people, what causes some researchers to suggest that this is just an adopted belief in nymphs, common among the Greeks. Second being would be topielitsa (lit. drowned woman) or the spirit of a woman who died by drowning, often as a result of suicide or murder and returns to take revenge on the humankind. The third might be East Slavic vodyanoi, vodnik (lit. water man) or shulikun, usually masculine and only vaguely humanoid being, commonly having pronounced piscine or amphibian traits, such as gills, bulging eyes or green skin, These beings were generally believed to be local 'caretakers' of water bodies and keen to scare people or even drown them to keep them away, but they could have also been reasoned and bargained with.

Depending on time and area, character and modus operandi of the beings that were most probable precursors to the modern depiction of rusalka could have varied tremendously. In Western Slavic regions, they were often associated with women who died in their youth or simply before marriage. In Ruthenia and Lithuania, they were often considered to be spirits of children who died without being baptized, while in northwestern Poland and Eastern Lithuania they were often associated with women who died by drowning, regardless of circumstances. In the latter region they were often depicted as old women, sometimes even with some signs of body decomposition, while in other regions they were described as young girls, often with long, loose hair (in late 19th century Western Russia one could have find common disparaging comment 'to keep one's hair loose like a rusalka'. In southeastern Poland and southwestern Kievan Rus (and later also Grand Duchy of Lithuania) such beings were generally believed to be playful, but also capable of tickling their victims to death, hence popular name like kazitka, shchekotukha or loskotukha (from kazitat', shchekotat' or laskotat', all meaning 'to tickle'). In southern Poland and Red Ruthenia such beings were known as boginki (ukr. bohynki) or dziwożony (lit. wild women, possibly from Czech/Slovak 'divoká žena') and associated with kidnapping of young children or swapping them with their own, in a local variation of the 'changeling' folk belief. Most often than not, they were believed to be spirits of women who drowned, died in a childbirth or committed infanticide. It is worth noting that all these creatures, especially in Eastern Europe were not necessarily aquatic, as they were also associated with deep forests (sometimes believed to control forest animals), especially in northeastern Poland, northern Russia and Byelorussia, while in relatively sparsely forested territory that is now Ukraine, such belief was obviously less popular.

The history of these differences is generally considered to be reaching back to the Early Medieval period, and between very little to no contact with Christian and Muslim world and lack of written sources made by Slavs, it is largely considered to be lost. Conjectures based on the material collected by folklorists since 18th century and later anthropological comparative analysis of such material are also not uniform. Some scholars favour the theory that the manous character of discussed beings (i.e. the latter being some form of human soul) came only with the Christianity and its belief in the immortal soul and pre-Christian Slavs were treating all supernatural beings as essentially completely different from humans. Others, pointing to e.g. Ancient Greek myth of Lamia suggest that the belief in malicious spirits being created by human actions might be pagan in origin.

This said, Propp's conjecture that rusalkas were benevolent nature beings that for some reason morphed into malicious aquatic spirits is disputable. First and foremost, even in early 20th century in the rural areas of Central and Eastern Europe, some people were still believing in nature spirits that could have been helpful if approached correctly, very possibly reflecting ancient customs of the pre-Christian offerings to deities or nature spirits. A lot of so-called 'field demons' were essentially nature spirits capable of influencing weather or harvest, such as Bielorussian bielun or zhytsien, Russian sparna or ray, Lithuanian laukasargas (not exactly Slavic, but the Lithuanian and Ruthenian folklore mixed during formation of the Grand Duchy of Lithuania over 13th and 14th century) or Russian rzhanoy dyed (lit. old man rye). In addition, Eastern Slavs were also believing in nature spirits guarding forests (e.g. leshy or lesovik) or controlling weather (e.g. Polish płanetnik), with such beliefs being well and alive even in late 19th century. Such beings were especially numerous in complex mythology of the Slavic people inhabiting northern Carpathian ranges, especially alpine Tatra mountains (now on the border of Slovakia and Poland), with the aerial spirits controlling wind, rain and clouds being very prominent, quite possibly due to the extreme weather conditions that can occur in the region. Thus, existence of such beings in local beliefs, as well as the ability to bargain with aquatic beings to secure safe swimming, travels of fishing also suggest that the transition mentioned in the beginning, even if actually happened, was neither universal, nor complete.