I think he's a Jinchuuriki. Actually, I think "he" is a Bijuu, and that he uses the Cursed Seal/Body Transfer techniques to change his Jinchuuriki. For one thing, he seems to have more chakra than his current body can safely use, leading to the degeneration discussed. Like you said, he's "naturally" very snake-like, which kind of points to Bijuu chakra, just like Naruto's Bijuu chakra makes him kind of fox-like. Also, Sasori planted Kabuto in Orochimaru's organization, which might indicate that Orochimaru was Sasori's target.Originally Posted by Yukimura
(On the other hand, it's possible that he's a Junchuuriki that does soul transfer or whatever and brings the Bijuu along. Or that the Bijuu warped his mind into something not quite human.)