Ancient Baatorians
From Marillia
According to the fringe writings of a handful of sages, the baatezu were not the first inhabitants of the Nine Hells of Baator. They claim that when Asmodeus first arrived, he found it inhabited by a primordial race of Lawful Evil fiends who predated the baatezu, tanar'ri, and yugoloths. Their true name has been long lost to history and myth, and as such they are only known nowadays as the ancient Baatorians.
We know very little about them, save for a handful of clues scattered across the multiverse. For one, it is believed that Zargon the Returner was their leader at the time of Asmodeus's arrival. We also know that the baernaloths claim to have created the ancient Baatorians. The nupperibos also may have some connection to them.