The book is named after him and we follow him from before he's born, so he has to be our protagonist. Plus, London takes special care not to judge White Fang for the harsh things he does, since he's a wolf and can't help it. But in his own wolf-y way, he goes through the same hero's journey that every classical hero does. He faces danger and threats in a strange land before finally emerging triumphant. We don't see many stories like that where the wolf is the hero (usually, they're running after the hero trying to eat him), but hey, that's just part of what makes this story so cool.