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 race of beings.
Ecology & Appearance
It is well known that Baatezu reproduce through the twisting of larvae into Lemures, "young" Baatezu who then ascend through the ranks. What is much less well known, 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, who the Baatezu supplanted and scoured from almost all records.
As you might imagine, this makes any concrete details on the Baatorians extremely hard to pin down. Sources describe them as formless creatures more akin to slimes or abberations than the humanoid shapes that most outsiders resemble, or as noncorporeal entities entirely, existing only as abstract concepts of primal terror, that manifest occasionally in the deepest darkest recesses of the lower planes. What is concretely known is 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.
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 when the Yugoloths purged themselves using the Heart of Darkness could instead be applicable to the Baatorians and the Obyrinth, and the myth has simply warped over time to remain applicable. Individual Yugoloths are remarkably unhelpful when questioned on this, as per usual.