Right after I read The Broken Path a year ago, I began to suspect there might be a connection between Old Hunter and Terror. In The Empty City, Old Hunter was suspicious of Lucky, even when he was a friend. Lucky wanted to partner up with him, but Old Hunter refused, since he was a Lone Dog at heart. Lucky also noticed that the Big Growl seemed to changed Old Hunter inside. And there's another thing; Old Hunter doesn't trust any other dog that came to challenge him for his food and also fought off a Pack of foxes. Doesn't that sound a bit like Terror?
In the next three books, Old Hunter is in the Pack List. While he is mentioned in books two and three, but he is not mentioned in book four. Meanwhile, Terror, who is not in the Pack List, appears and threatens some of the Wild Pack. Terror even attacks one of his Pack members.
"Where have you slunk from? Strange dogs are not welcome here. The King of Dogs does not share his prey!" - Terror before he attacks one of his own dogs, The Broken Path, pg 115.
Why exactly would Terror attack one of his own dogs? Is it because he doesn't trust them like Old Hunter wouldn't trust any dog, even Lucky? And there another thing: If Old Hunter is Terror, then how come Lucky doesn't recognize him? Something had obviously happened to Terror: he has patches in his fur, his eyes are wide and dilated, and he even has seizures. Here's on article on dilated eyes: http://www.healthgrades.com/symptoms/dilated-pupils
I'm jumping the boat here, but what if Terror somehow escaped from the yellow longpaws? If he ever did, the longpaws must have done something to hurt Terror. The longpaws must have injected Terror with something that made his eyes that way. They must be the same longpaws who captured Fiery and tested him and other animals. This is definitely animal testing, if nothing else.
Terror also seems to be frightened of something after his seizures. Could it be the "memory of the longpaws?" Could even Terror be scared of the Fear Dog? And how did Terror come in touch with the Fear Dog?
To be honest, this isn't a full analysis. In fact, this is not what a real analysis would and should look like. There is a lot more I could've put in, I might even be missing some important details. And the thing is probably a big mess. Maybe it's better this way, to keep this a mystery. For now.