You'd be surprised how often horror writers go back to the same few wells for demon lore. The Ars Goetia from old grimoires like the Lesser Key of Solomon keeps showing up—demons with elaborate hierarchies, specific powers, and seals you're supposed to use to bind them. Authors love that stuff because it gives a built-in system of rules the characters can discover and maybe exploit. Also common is the fallen angel narrative from Christian tradition, but twisted: not just rebels, but entities corrupted by their own desires or by exposure to human evil. Sometimes the 'true demon' isn't a religious figure at all but a manifestation of a collective human sin, a concept I've seen in a few quieter, more philosophical horror novels. Makes you wonder if the scariest demon is just us reflected back, but way uglier.
What I find creepier, though, are the ones built from older, weirder myths. Some authors pull from Zoroastrian dualism, where the demonic is a fundamental force of chaos and destruction opposed to order. Others dig into pre-Christian folk beliefs about land wights or household spirits that turned malicious because they were forgotten or offended. Those feel less predictable than the standard Catholic-exorcism template, and the rules are stranger, which raises the stakes. The demon in 'The Cipher' by Kathe Koja kinda fits this—it's more an amoral hole in reality than a dude with horns, and that ambiguity is way more unsettling to me.