Accounts of the Pact Primeval and Asmodeus' first arrival on Baator say that it was a featureless plain, devoid of inhabitants — but it's well known that devils are liars, and their claims are contradicted by scattered evidence that Baator was once home to a completely separate kind of beings.
Ecology & Appearance
It is common knowledge that Baatezu reproduce through the twisting of larvae into lemures, "young" baatezu who then ascend through the ranks. What is much less known, however, is that larvae left untransformed on the plane instead transform into nupperibo. Interestingly, this is also the default state in which Devils killed outside of Baator reform into. The Baatezu pass this off as a form of dead-end demotion, a disgrace for dying - in truth this may be a natural effect of the plane, for the nupperibo are supposedly the larval form of Baator's original inhabitants, whom the baatezu are said to have supplanted and scoured from almost all records.
As you might imagine, this makes any concrete details on the baatorians, if they did exist, extremely hard to pin down. Most sources describe them as formless creatures more akin to slimes or abberations than the humanoid shapes that most outsiders resemble, or even as entirely noncorporeal entities, existing only as abstract concepts of primal terror that occasionally manifest in the deepest and darkest recesses of the Lower Planes. They agree, however, that the baatorians were extremely hard to kill, possessed of great regenerative properties; traits that are still evident in nupperibo, forming another link of evidence. Some few of the sources suggest a resistance to divine and scrying magic, perhaps explaning why fiends reformed as nupperibo must be located manually, and perhaps the reason why Asmodeus had been banished to Baator in the first place.
Their creation and purpose is likewise murky: one theory that has gained traction among scholars of Fiendish History is that the myth of baatezu and tanar'ri originating from the runoffs of Law and Chaos from the yugoloths' purge by the Heart of Darkness could instead be applicable to the baatorians and the obyrith, and the myth has simply warped over time to remain applicable. Individual Yugoloths are remarkably unhelpful when questioned on this, as per usual.
It's hard to pin down whether any of these supposed creatures currently exist - the apparent long "gestation period" for nupperibo means observing a creature for the time needed to mature is impractical, though such a drawback would not deter the yugoloths and their arcanoscientists, given that they are immortal and (as a rule) extremely patient. The yugoloths also engage in sporadic trading of nupperibo, similar to the larvae trade between night hags and fiends, even though there are much more efficient trade goods and food stores.
The most likely places to find baatorians, of course, would be in Baator, even though it is filled with Devils nowadays, and resembles a giant military base more than a Plane. Any potential civilisations or creatures would be hard-pressed to exist under the cast-iron glare of the baatezu, though there are a few places and phenomena of interest.
Adventurers and miners within the deepest recesses of Maladomini have reported shapeless creatures, indestructible masses that breathe in light and breathe out darkness, with some of the more aggressive examples breathing in life instead of light. Similar rumours float around Malbolge, and the terrible screams that occasionally echo from deep caverns around the layer - though oddly only tieflings and fiends appear able to hear them.
More observably, a canny cutter can sometimes spy strange things in the glaciers of Cania: snapshots of battles between baatezu and archons, sometimes with unrecognisable creatures caught in the fray. Of greater interest to scholars are the buildings, ranging from simple to elaborate, all seemingly carved or shaped from solid stone, and almost all incorporating alien geometries. Planar merchants extracting frozen commodities from the Paraelemental Plane of Ice have reported similar phenomena, possibly contamination from crossplanar connections originating from sufficiently large Canian glaciers; one especially crackpot theory is that the Sleeping Ones are in fact ancient Baatorians (or perhaps even their gods).
The last and least searchable place where Baatorians might be found is on the Prime Material Plane itself. Scattered accounts of the supposed ousting of Baator by the baatezu say that some of the ancient evil fled to the farthest reaches of the Prime and buried itself, or was thrown there by Asmodeus and his army of devils. So far expeditions to the Prime have proved fruitless - there are simply too many worlds, and scrying has continued to be ineffective in locating signs of baatorians. The most promising lead so far is a cult from the either the prime world of Oerth or Toril - accounts vary - dedicated to the worship of "Zargon the Returner", though this cult seems to have vanished from all records.
- Elder Evils, pp.144-148
- Fiendish Codex I: Hordes of the Abyss, pg.107
- Faces of Evil: The Fiends, pp.12-14
- Hellbound: The Blood War - War Games, pp.74-75
- Manual of the Planes (3e), pp.121-122
- Tales from the Infinite Staircase, pg.127
- The Inner Planes, pg.72
- The Lost City