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?