Being that I'm half Japanese, an amateur historian/folklorist, and somebody who was largely disappointed with Oriental Adventures, from back in the day - so many mistakes and/or concepts taken out of context, and the inaccuracies regarding Japanese monsters, I knew that someday I would try to fix those. I sought some measure of historical/folklorist accuracy. So from 2010 - 2017, I developed and published the
Kaidan setting of Japanese Horror (PFRPG), 15 books and supplements - I started development after 3.5 ended and before 4.0 began, hence why it's for Pathfinder. But it's a setting, with only some Pathfinderisms in there, and could easily be run in 5e with some tweaking. While we never came out with a full bestiary on Japanese monsters, the adventures (3 full modules and 4 one-shots) feature many Japanese monsters which were developed closely with Japanese folklore for accuracy.
The Kappa for instance probably exists for 5e, it definitely exists for PF, but at the PF version was waaaay off. Kappa were known to be expert wrestlers and bone-breakers, and included a paragon racial class for Kappa called the bone-breaker, for example. I was also using the pencil art of Mark Hyzer for all my monsters, which has a nice quality about them.
Of course I included lots of monsters that never existed in RPGs before, like the Suragami. An aberrant being that resembles a disembodied horse's head with a tail for it's "neck". This monster kills riding horses out of human watch, eats it's head off, then inserts it's snake like body into the cavity of the neck where the head had been. It's magic then animates the dead horse, and hides the decapitation wounds, awaiting for it's rider to get on the horses back. While along the trail, whenever the rider gets out of eyesight of his allies, the Suragami attacks. It pulls itself out of the neck, which immediately causes the animated horse to loose animation and fall to the ground trapping it's rider's leg under the weight of the dead horse, then the Suragami strikes at the rider attempting to bite his head off, and consume the rider. Pretty nightmarish stuff!
