Kongamato

The name means the breaker of boats, and it is a job description rather than an insult. It comes down the river at head height with a wingspan wider than the channel and takes the canoe rather than the men in it, which sounds survivable until you consider where you are and what is in the water. Boatmen on that stretch carry a charm for it and will not sell you one.

Breaker of Boats: bludgeoning, melee 10 ft, +4 to hit, 2d8+3, and it attacks vessels, rafts and rigging in preference to creatures, always. A struck vessel is holed and begins going down. Beak: piercing, melee 5 ft, +4 to hit, 2d8+3, used on anyone still aboard after the boat is gone.

On the Wing: it cannot be reached in melee while airborne and does not land during a fight. Ranged weapons, thrown spears and reach from a mast are the only things that touch it. Loses Interest: with no boat left to break it circles once and goes, whatever its remaining HP. It has never once pursued swimmers, which is fortunate, because there is something else in that river.