The Monster of Vlasinsko Blato is a cryptid that is thought to have lived in the peat bog known as Vlasinsko Blato which existed at the site of todays Vlasina Lake before 1954 when the bog was turned into the lake. The creature was described as looking like a giant horse. It would steal cattle and drag it into the mud. The origin of the legend is unknown, however it is possible that the legend originated after an entire Bulgarian cavalry regiment dissapared while trying to cross the bog during WW1. In reality the regiment was probably swallowed by the mud. It is belived that the monster died when the bog was turned into a lake.
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Note: source is in serbian