Take a head, give it crab legs and stone teeth, and you have the dossenus. At CR 1, it doesn’t seem like much of a threat. But given that two can occupy a single space (and are considered flanking), and that an infestation can involve 20 of the creatures (that’s not even counting when they swarm), they suddenly get a bit scarier. Plus those teeth can tunnel through stone and act as adamantine in terms of overcoming damage resistance and hardness, so hiding isn’t an option. At least they're afraid of water…but in the Realms Below, water is so scarce that using it as a refuge just means handing yourself over to something worse…
(As a bonus, they speak Aklo and cannibalize their dead. That’s never a good sign in your subterranean horrors. Oh, and their religion involves eating all other forms of life. This may make me sound prejudiced, but…yikes.)
Let’s address the elephant in the room, though. Dossenuses don’t feel like Pathfinder…or at least, they don't feel like Golarion. They don’t even feel like D&D’s modern Underdark. They feel like a throwback to 1e AD&D, before the Dungeoneer’s Survival Guide began to codify Deepearth into the Underdark/Darklands we know today. They read as Fiend Folio monsters, not Bestiary monsters.
That doesn’t mean I don’t like them.
Ages and ages ago, I wrote about gugs and their place in an Uncertain Underdark…a weirder place that the Darklands we know today:
[A] more 1st Ed., pulp-era world of dark caverns and lost, strange cities and weird, alien monsters.
And I do mean weird. Look up Dragon #281 (this isn’t something I usually encourage, but you can find a PDF in seconds if you Google) and check out “Subterranean Scares” by Joseph Terrazzino. In a world of two-headed jawgs, snake-vomiting genocids, and verx swarms, gugs fit right in.
And so do dossenuses. I don’t know if I want them in my usual Pathfinder games—I’ve got shriezyx for that. But I sure as hell want them in my unusual Pathfinder games.
Adventurers are exploring deep underground when their path dead-ends at an opening overlooking a small, crystal-lit cavern. Below, dossenuses are holding a funeral ceremony, consuming three of their kin. If the adventurers are spotted, the aberrations abandon their rite to honor their god by devouring the living. Adventurers can fight or flee across a nearby river, but that second option puts them in the domain of a lampad, one of the strange weeping cave nymphs.
A street preacher worships the Graveyard God in one of his ancient, obscure aspects, the Eater of the Dead. Working in secret, he opens a passage to the caves below the city, and dossenuses come boiling out to consume new flesh. The preacher is the first to be consumed, but whatever scraps are left over begin to coalesce and grow, animating as a hungry flesh. Whether this is the god’s reward or punishment is hard to say.
Captured by duergar, adventurers face a lifetime of slavery. Then a drow army attacks the city. Being surface dwellers, they are allowed by the armies to serve as medics and message runners in situations where a gray dwarf or dark elf would be executed as spies. Just as they begin to settle into their roles, the warring armies are forced to unite against a new threat—hundreds and hundreds of swarming dossenuses on the march. Since neither army trusts the other, the surface adventurers will have to lead the defense.
—Pathfinder Bestiary 4 63