I don't know why I didn't think of it sooner.
Why don't elves in the Beuttleria setting retreat to the forest? They know what comes out at night.
Why don't dwarves in the Beuttleria setting live underground? They know what lives there.
The underdemons from the dark heart of the world. Black scrabbling chitinous serpents. Sleeping now, but once woken, they will purge the world.
Ok, so I'm taking the basic framework and stats of kythons, but making some modifications. I'm including some of the traits of Crabs from Spots the Space Marine, reducing their sentience, dropping the Infernal/Abyssal as languages (they aren't actually demons, that's just something the dwarves call them.) I'll probably remove the complete immunity to heat and cold, maybe make it 50%. Of course, with no magic in the system, PC's will likely just be responding with blades rather than fireballs, so leaving the immunities isn't a big deal.
I'm not calling them kython in the game, I'll just be using cultural names for them from the races already there. I haven't decided what period of history I want to introduce them in, whether I want the colonization itself to awaken them, or perhaps a Beisili or Beuttlerian mining operation.