I think it's a safe assumption to make that Barbas is Clavicus Vile, at the very least, to some extent. But how? And why? No other Daedric Prince has ever appeared as two seemingly separate entities at once, as far as I know. And why are they constantly at odds? It seems as though Barbas wishes to fight against Clavicus Vile's cruel side.
It's known that Barbas almost always appears as a dog, but he can and has changed forms rapidly and unpredictably. That's a trait all Daedric Princes have, though they rarely seem to use it outside of Boethiah changing genders at will, or Azura getting some more clothing. Barbas, however, has been seen rapidly changing between one shape and another multiple times per minute. In Redguard, when Cyrus meets Clavicus Vile and Barbas, Barbas even http://www.imperial-library.info/sites/default/files/tsorg_part10image12.jpg'. And then, there's the idea that perhaps Barbas was http://www.uesp.net/wiki/Morrowind:Creeper at one point, the Skamp making deals with the Orcs in Ghorak Manor, in Morrowind.
Clavicus Vile comments on Barbas:
He's not a Dog, of course. On sunny days, he likes being a Dog. Or lots of other things. When he's like this, I think he's somewhere else. It's all quite significant, of course... But who knows what it means.
What does it mean? Any theories? Speculation?
(On a slightly unrelated note, I really miss Clavicus Vile's voice from Redguard. It was perfect, and I always try to pretend its still that dual-voice thing going on when I talk to him in Oblivion or Skyrim, and its the voice I heard in my head when I read Lord of Souls.)