Manual of the Planes (2001) says
Though the Nine Hells is governed by devils, some sages believe that the devils commandeered the Nine Hells from a far older, stranger race now simply called ancient baatorians. It’s possible that remnants of this mysterious race still inhabit isolated portions of the Nine Hells. (116)
Further, Malbolge, the sixth layer of the Nine Hells, on the cavern Maggoth Thyg says
Rumors describe ancient places built underneath the slopes of Malbolge. Below the hellish rock and stone, creatures older than the devils themselves—the ancient baatorians—might still roam. (121)
Did Wizards of the Coast ever describe the ancient baatorians, and, if so, what are they?
Best Answer
Wizards of the Coast did not ever describe the ancient Baatorians
While one of the writers of the book Elder Evils did attempt to jazz up the final entry in that book by naming Zargon the Returner as an "ancient Baatorian," it should be noted that this same Zargon was earlier detailed in Dragon 315, where he was a Native Outsider on the Prime Material Plane. In the original module The Lost City, Zargon's alignment is given as Chaotic.
Zargon is the only entity in 3rd Edition D&D explicitly identified as an "ancient Baatorian" (their supposed leader, no less), and so is by default the only footing we have from official WotC material regarding what an ancient Baatorian might be. However, Zargon as depicted in Elder Evils does not align with older information regarding the ancient Baatorians. If you are looking for the official word and no further, then that's all there is. My personal recommendation would be to disregard Zargon entirely in this matter.
...but what are they?
As an element of flavor in D&D, the ancient Baatorians originated in 2nd edition as part of the Planescape line. They are most commonly understood to be the native entities of Baator, who vanished long before Asmodeus and his ilk descended into the Hells and whose rule in that plane has largely been supplanted by the baatezu. They are attested as having "ascended" beyond physical forms in some way, becoming formless evil thoughts. Not all of the baatorians have transcended, mind you - their "young," the nupperibos, still pop up where larvae adopt the bloated and horrid form that starts the journey through the ranks of Baatorian kind. The baatezu fear the possibility of nupperibos maturing to become more advanced Baatorians, and ruthlessly round up the fat fiends to render them down into lemures, the base state of the baatezu race.
One nupperibo was secreted away and kept imprisoned, its jailors hoping to have at their disposal a matured Baatorian with which to menace the devils. The early moments of its maturation are depicted in this image. This encounter features in Tales From the Infinite Staircase.
The dogai, or assassin devils, are said to have been crafted by Asmodeus from malignant living shadows native to the Hells. Feared and mistrusted among devilkind, the dogai may have originated as ancient Baatorians - their description as "sentient, malicious will" certainly evokes the "ascension" of the Baatorians.
Deep within the ice of Cania, terrifying shapes are bound up in an eternal tableau of conflict with celestial beings. The fiendish combatants are believed to be ancient Baatorians.
Baatorians have been attested underneath Malbolge and Maladomini. In the latter case, a Baatorian is actually encountered in an adventure from Hellbound - a creature of "anti-light" that sleeps beneath a city, it is capable of devouring not only light but also "lightedness" - the capacity to be visible - by sucking this quality out of anything that strays too close. If awoken, it idly consumes the life force of those nearby for a few rounds. This Baatorian is noted to be virtually invincible - only a wish spell will faze it. This bizarre encounter is depicted here.
Drawing from these adventures and the information they provide about the Baatorians, we can infer that Baatorians were an ancient race of fiends who likely possessed immunity to fire, cold, and electricity (the matured nupperibo from Tales) and who tended to possess a noteworthy ability to devour energy, especially life energy (both Baatorians as described above). Beyond that, we have few meaningful clues, as was doubtless the intention of those writers who slipped this into the setting.