Jump Sharks

got me a conquistador

Freshwater sharks that live in swamps and have evolved fins like flying fish. They can jump across narrow strips of land to bite/tackle victims and drag them under the water. Their fins are as articulate as those of a bat, so they can glide quite a distance before transitioning into a football spiral that takes you head over heels when they grab you in a terrifying surprise judo drowning bite attack that ignores armor (so combined with the penalties to swimming in heavy armor pretty much mean don't wear full plate mail in the fucking swamp but players will probably do it anyway).

They know where the water is because they've learned the labyrinthine geography of their swamp completely. Not in an individual sense, though they've been swimming these waters all their lives, but in an evolutionary sense.

Cab drivers in London have to take an extensive test for which they must learn the entire layout of the city, and the process of learning this intensely complex pattern actually changes the physical structures of their brains. Jump sharks have it the other way around. They're born with the layout of the swamp where they evolved hardwired into the frontmost part of their brain.

This means that if they're somehow removed from that environment, their normal hunting methods are useless. If they can still manage to survive on a purely aquatic diet, their descendants several generations down the line (let's say 80 years -- they have unusually short lifespans for sharks) will have adapted their brains to their new home. They're usually a bit smaller by this time, but the sub-species created by this process tend to return to their normal size pretty quickly once they've reincorporated the land into their hunting grounds.

Of course if boats are commonly used in the area all bets are off.

Characters with high enough intelligence have a chance of learning the layout of the local region, including hard-to-spot bogs and quicksand, by carefully inspecting the brain structure of a dead jump shark. If an intelligence check fails, a read magic spell will still do the trick.

They know where you are because they've also evolved eyes that can detect the infrared radiation produced by humans and other warm blooded animals. This even applies to newly transplanted jump sharks that can't get at landswimmers at all anymore, which pisses them off to no end.

siberian bear hunting armor

Your creepy neighborhood swamp hag may be able to use these sensory organs to craft items that grant their users infravision, in exchange for some favor that should probably take at least a couple sessions to deliver on. Anyone who can practice normal alchemy has a chance of figuring out how to distill them into potions that (temporarily) have the same effect.

The only warning sign of their presence is their well-camouflaged dorsal fins, since they must swim close to the surface to get around the water's ability to absorb all radio waves. You and I can't see as far in water as in air because it absorbs visible light waves, and infrared waves work the same way.

The only type of armor that's actually useful against them is the kind with pointy spikes all over it. Any nearby cultures with at least Bronze Age technology are almost guaranteed to have figured this out, so it should be available if they exist.