Actually zmaj/dragon is just a male snake/zmija. Zmaj-zmija. Zmija and zmaj, dragon, snake, have deep roots in Indoeuropean past. In all myths among Indoeuropeans you have some kind of big snake. On the bottom of tree or mountain or near water there is always a snake.
@Perun Veles is sometime depicted as bearded snake (zmaj, because snake and zmaj have different conotations in Slavic languages and English). It is all about myth about cosmic fight between Perun and Veles. Perun: dry, hill, mountain Veles: moist, river, water These are areas where Perun and Veles could be “found”, to be exact their area.
@Sviatogor Among East Slavs St. Nicholaus or sv. Nikola “took” Veles traits. Uspenskij wrote about that. 16 km of Novgorod you have Veles temple who became Волосово Николаевский монастырь.
Among West Slav: Ky jest črt, aneb ky veles, aneb ky zmek te proti mne zbudil? “Which devil, and veles, and dragon rebel against me?” Czech roman Tkaldeček, 15. century. Here you have Devil, Veles and dragon as one thing, same thing.
And there are other examples all around Slavic world
@Sviatogor Do you know any legend in Belarus in which is mentioned змея камень and how it was strucked with ПЯрун?