I definitely do not think its the other Sound Nins. Both of them were injured to the point where they were forced out of their level 2 curse stage. That means even if they did regain consciousness or whatever, they would barely be able to move. No chakra. Also, they would be absolutely worthless in a fight. I've got to agree that its someone he's afraid of, and I mean seriously afraid of. The only person I can think of who creates that much fear in Kiba is Gaara. You have to remember that Gaara has been training as well. Gaara is another that wants to be the strongest, and since he lost, he will definitely train to become stronger.