Mokele-mbembe

It is enormous, it lives in the deep river bends, and it eats plants. It kills hippopotami and does not touch the bodies afterward, which is the detail everybody fixes on, because it means the killing is about the river rather than about hunger. It has no interest in people until a canoe crosses a line only it knows about. It has never yet been the aggressor in any account that survives.

Overturn: bludgeoning, melee 15 ft, +4 to hit, 3d6+3, aimed at boats and at anything large in its water, and it turns them rather than breaking them. Sweep the Tail: 20 ft, save Dex Quick, fail save roll 3d6+3 bludgeoning, cooldown 3 rounds, used to clear its stretch of river rather than to kill.

Not Hungry: it eats only plants and will not pursue anything out of the water or onto the bank. A party that gets ashore is finished with it. Territory, Not Malice: it stops the instant its stretch of river is clear, whatever damage it has taken, and it will not follow around the bend. It is a hazard with an edge to it, and the edge is on a map somebody could have drawn.