Arapaima

The breath of an arapaima sounds like a heavy cough across still water. Beneath it moves a river fish as long as a small boat, armored in overlapping scales and forced to surface for air. A nesting adult strikes paddles, swimmers and narrow craft from below, using its broad head and body to drive threats away from shallow spawning ground.

Surface Ram: bludgeoning, melee 5 ft, +3 to hit, 1d10+2. A swimmer or occupant of a canoe-sized boat makes a Quick save; on a failed save, the creature is thrown into the water or the boat is driven 10 ft off course.

Armored Scales: the arapaima has DR 2, but attacks against the mouth or gill opening ignore that DR when the fish surfaces to breathe. It must surface once every 10 rounds, revealing its location before its next turn.