River Monsters (that are not fish) and Monster Rivers
There are seven seas, but seven million rivers. Each one has its own personality, history, culture of communities that have built themselves around it.
Rivers are almost always nearby. Every populace needs a source of freshwater, and rivers are an obvious one of those. So if some big and belligerent monster takes up in the river, it's an easy way to get those lazy PCs adventuring. You want EXP? Go down to the banks and hike a mile. You'll get some.
River Monsters
The Shield-Hydra
A shelled reptile the size of a cow, with squat limbs, a slow gait, and -I cannot emphasize this enough- 2d4 serpentine heads, each with the head of a snapping turtle. The creature likes to bathe in murky water, snapping at birds that try to settle on its back, or eels that swim by. It is as venomous as its bare-backed cousin, though far slower to move its limbs- the heads are still whip-like though, so have fun with that.
Each head has 2HD and AC as leather. Bite as medium weapon, save vs poison. If a head is killed, it withers and two more heads grow in its place.
The body has 4HD and AC as plate. Move at half speed.
The River Salamander
A wagon-sized axolotl the color of river muck, with a mucus-lined skin and a very distinct odor. These sedentary, largely docile creatures are easily spotted by the oily, viscous secretion that they produce when in water. A grown salamander can produce up to 5 liters of this sludge a day.
When the River Salamander is threatened, it may remind you why it is called a Salamander, and cause its secretions to burst into greasy flames, spreading across the surface of the water and leaping onto anything nearby. Unlike ordinary fire, these cannot be extinguished with water, and burn exceptionally quickly. The Salamander itself is seemingly immune to fire, though not immune to being on fire; it will trudge along for hours after an ignition fully ablaze, burning down swathes of wilderness by sheer happenstance.
4HD, armor as leather + shield. Attacks as medium weapon.
Lancing Heron
This large, graceful bird seems innocuous at a glance, a typical specimen of its species. That is because you haven't seen its nest yet.
In the high and bare boughs of the trees is a reeking nest of human hearts, each one punctured cleanly through. Hatchlings fight one another for the fleshy morsels, tearing it apart with their beaks as soon as they are firm enough.
Once the heron senses a target, they will at first seem evasive, flying up and away... before diving back at the unwitting target, their beaks honed in on the heart.
2HD, unarmored. Attack as spear, additional +2 to hit if it has moved at least half its movement in a straight line. Targets dropped to 0 HP by this must save or have their heart ripped from their chest.
Otterconda
A twenty-foot-long, serpentine otter with scales beneath its fur and heavy, dense teeth that curve like snake's fangs. It is the end result of generations of swamp-witches throwing their familiars into black cauldrons together without thought to the consequences. The creature is territorial, often coiling itself around the rocks of the riverbanks.
3HD, armor as leather. Bite as dagger that reduces the AC of rigid/metallic armor by 1. Can attempt to constrict for medium damage (target may save).
Monster Rivers
The Boiling Rapids
A wide but shallow span of quick-moving water that is perpetually at a low boil. Standing in this water for any amount of time deals damage, ruins most armor, and can scald flesh. The shores are largely bare of plants, and most animals give the place a wide berth.
Most animals.
The local populace of crawfathers, gargantuan crawdads as large as apes, are perpetually trying to perform more and more impressive feats of strength and resilience to woo potential mates. Once they deduced that these waters were hard to withstand, it became a trial of theirs to bathe in it as a show of force. At any given time, there are 1d6 bright-red crawfathers skittering amongst the water with half HP, doing battle with one another.
The Dragon River
Long ago, great dragons of the sky saved mankind from a drought by allowing themselves to become mighty rivers, giving water and fertile soil to generations.
This river is slowly turning back.
The twisting bends of the river slowly coil and uncoil over days and weeks. The sound of the rapids sounds unmistakably like a roar some days. When the sun hits its zenith, the water's sparkling is unmistakably in the pattern of scales.
Druids have communed with this river and learnt that it is tired of being fished from, polluted, and dammed over and over. It intends to slowly reshape itself and take for the skies again. The countless villages along its shores are panicking, and willing to consider any options to keep their blessing where it is.