saw a claim that a lake dragon STOPPED the Mongol invasion of Novgorod, need a sanity check

求 anyone who knows Russian folklore — somebody on another board dropped this as fact and i can't tell how much is real legend vs them embellishing. Lake Brosno, Tver region, ~250 miles from Moscow. supposedly home to 'Brosnya', a dragon thing, fish-head, long tail, eats men and boats whole. the bi……