Actually, Graz'zt is very much like folkloric figures Krampus, Zwarte Piet, the Black Man of the Woods (of Salem witch trial infamy), Old Scratch and others. And Erik Mona's Armies of the Abyss introduced him (sorta) as a "familiar" demon lord with the modified name of... Vaz'zht or something similar.
I agree that it's not quite as cut and dried as Orcus, but you could get there by remixing various existing OGC and public domain folklore, although granted it wouldn't be exactly the same. It would be as close as you were able to get to the D&D versions of Dagon or Malcanthet, though.
That's certainly true about Graz'zt... he draws upon a LOT of those types of traditions... but so do a lot of other demons. There's no "Graz'zt" in mythology, nor any ebon-skinned six-fingered wavy-bladed-bastard-swording mythological figure out there that he's specifically based upon, and so we can't and don't touch him. We've got our own characters who fill this role, of course, with Socothbenoth being the primary one.
Orcus is, in fact, entirely open content in his classic D&D incarnation thanks to the Tome of Horrors. We've not done much with him in Golarion mostly out of respect for the fact that he's shown up as a major player a LOT in various D&D books AND in various Necromancer Games/Frog God books, and more. Put another way... Orcus has had plenty of time in the spotlight. Since he IS open content, we do have him in Golarion and he did get an entry in "Book of the Damned 2," though. We might do more with him some day, but I personally kinda scratched my own Orcus itch with Savage Tide.
Dagon is also in Pathfinder already, and his incarnation is relatively close to the version I helped build for D&D. Dagon's the perfect storm in a lot of ways... he had open stats in in Tome of Horrors, an established real-world mythology, AND was a significant character in the public domain from Lovecraft's stories. He's probably the MOST "open" of the various demon lords as a result.
Malcanthet, on the other hand, was pretty much created entirely by myself and Rob Kuntz for D&D. She does draw upon a lot of the classic succubus/Lilith type tropes, but that specific incarnation of the trope is off limits. Nocticula is the Malcanthet figure of Golarion, although she's got a lot more assassin and shadow stuff than Malcanthet did.