Well, it was not the only confusing thing in this mess, but some points they at least tried to explain to some level. But the dog who was there some times, even ate from his hand, but wasn't anywhere at next moments. There was no dog in any scenes with puma or when he fell into river... Was the dog also an imagination as I suppose? If it was, what was its point of being there?
shareEven if it wasn't there, believing it was helped him keep going, kept him from just giving up like he might have if (he believed) he was absolutely alone out there.
shareI believe it was a hallucination of his childhood dog to keep him going, to assist with survival. He even told the dog that he looked like his former pet "Duke." Also, toward the end, when Brody is climbing the last of the hillside, he speaks to some unseen character to his left, which was likely the dog.
It's implied that "Duke" was a mysterious guardian-dog-angel or the way Brody's mind was dealing with his mental survival needs. It gave him the strength to go on. A beloved, loyal childhood pet that loved him unconditionally was the perfect invisible friend at that low point.
At least, I think that's what the writer had in mind. :)