Kappa

Keep an eye on the water, lady. Pretty hair like yours really seems to drive the Kappa mad and I don't think I'm young enough to dive in after them.

-Said to Me by a Yuhin boatman as he sailed me across a lake.

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Even while Yuhi and Nianqi are forced to fight off the Oni at all times, they are not spared from the general ravages of the world, and in this particular case; monsters.
The kappa is one such monster, though it is perhaps less dangerous than might be expected of a creature that bears the title of 'monster'. The Kappa is an ambush predator, with its preferred method of hunting being similar to that of an alligator, with less focus ona bite. Kappa's will make their residence inside a large body of fresh water, and prowl beneath the water's surface where none can see them, but where they can see everything.
Should they sense someone near the water's edge, or god forbid sailing across its surface, the kappa will then strike by leaping out of the water directly towards the target. The speed of the little creatures is well known and even the most dexterous struggle to completely avoid their strikes when they don't know they are coming. While the Kappa has sharp claws and teeth at it's disposal, which it makes judicious use of to weaken it's prey, but very rarely to outright kill them. Instead, it is all an attempt to drag their prey into their watery home and drown them, where they will then be consumed. 
To this end, Kappa usually hunt in packs that can grow significantly in size if left unchecked. Many large bodies of water within the Land of Jet and Jade are considered to be "infested" by large amounts of Kappa and either aren't traversed by boat at all, or only with armed guard.

Geographic Distribution

• The Land of Jet and Jade

In terms of materials that can be pulled from the Kappa, there are far better choices even with monsters that exist within the Land of Jet and Jade, but they do contain a particular fluid within their skulls that seems to be a refined form of water aligned Aether that has some value when made into medicines and reagents.