Board Thread:Lore Discussion/@comment-77.172.125.107-20130831123130/@comment-3186827-20131001121842

Virtually nothing is known about the "old gods" aside from that they aren't the Atmoran Divines. They specifically take action in Skyrim to corrupt and profane the works of Dibella, and their reverence of Hagravens demonstrates a fundamental opposition to Kyne. Most people theorize Hircine to be one of their revered deities because of the anthropomorphically-corrupted origin of Hagravens as well as their animalistic hunter nature.

Personally I also believe Molag Bal is worshipped in at least some of their circles due to there being a strong presence of sadism, torture, and necromancy present in the Forsworn locations, as well as a shrine to him being buried beneath the modern city of Markarth, which was not under Skyrim's jurisdiction until Tiber Septim took the Reach in the late Second Era, and briefly fell back under Reachman control after the Great War in the rebellion that marked the birth of the Forsworn faction.

As for the rest of the gods, they don't necessarily have to be daedra. They could be powerful ancestral figures separate from the known aedra who have simply been forgotten by modern Imperial history, if they ever knew about them to begin with. It's likely they were powerful and violent dark mages that deeply ingrained those themes into the culture just based on how you see most of the Forsworn tend to behave.