Vodyanoy
In Slavic mythology, vodyanoy or vodyanoi (Russian: водяно́й, IPA: [vədʲɪˈnoj]; lit. “[he] from the water”or ‘watery’) is a male water spirit. Vodník (or in Germanized form: Hastrman), in Czech fairy tales, is the same creature as the Wassermann or nix of German fairy tales. In reality, vodyanoys are frogmen from the Age of Myths. Lyagubog is an ancient Slavic god, long forgotten by mankind, but in that age, like Ophidian, another anthropomorphic deity, Lyagubog existed in the physical world and could change his worshippers into frog people, in his case via kissing or through touch. Like Ophion, he was eventually attacked by a band of heroes, aided with help by Baba Yaga, and defeated. Like other gods of the Physical World, he was virtually unkillable, but was sealed in a bubble under Vasyugan Swamp where he remains to this day. His cult, the Cult of Warts, was completely decimated, but a handful of cultists survived. Many of these frog demons, the Lyagians, fled from their homeland in what is now Russia.
Over time, as the god was forgotten, so too were the tales and names of the monsters. Eventually, these demons grew weaker with each generation, until now they are much weaker and less intelligent than they once were. More akin to mere monsters they are called vodyanoys, and stay in wet places. They can exist in fresh or saltwater. They will wait for prey for hours, holding still just under the surface of the water. When a human approaches the shore or dock and looks over, a thick, meaty tongue will lash out, sticking to them. The vodyanoy will then pull them into the water in an instant where they are consumed. If they are not hungry, and desire to mate, they can excrete a substance, a slime, from their body that works as a microbial transmutater. The infected human will become a vodyanoy and help propagate the species.