I've always been fascinated by the universality of Myth. You know, the kind of stuff Joseph Campbell does. Full disclosure: I have a degree in History, with a strong focus on religion, but I am NOT an anthropologist, and more importantly, I've been reading the stuff below for fun, not research.
I stumbled on an interesting legend in GURPS Shapeshifters. This old Italian village had a tradition of the benandantes, or ‘good walkers.’ We know about this tradition because the Inquisition investigated it. Some of the peasants insisted to the Inquisition that they were ‘good witches.’ The Inquisition had them whipped for heresy, which was actually kind of nice, given how the Inquisition usually dealt with witches.
Benandantes were nocturnal shapeshifters. They turned into wolves as they slept. Rather than run around eating babies, though, they were good guy werewolves. Every night the Devil and his minions tried to steal the village’s harvest. But the benandantes wouldn't let him. These werewolves fought off Ol’ Scratch with iron whips (a delightful detail, that last one).
Today, scholars believe this tradition was a survival of a pagan fertility cult into the Christian era.
It has a lot of parallels. Lithuania had good guy werewolves in their stories, too. This reiterates a common belief among scholars that the evil werewolf is a Christian revision of an earlier legend. It was common for Christian storytellers to take pagan gods or heroes and twist them into villains. It’s worth noting, however, that the opposite also happened. They would also take "pagan" heroes and turn them into Christian heroes. (Pagan, by the way, is an insult, the Roman equivalent of 'hillbilly' or 'redneck').
Across the Pond, the Mayans had a strong corollary to the benandantes. When Mayan shamans (a term of convenience here) entered the spirit world, they became animals called wayob.
Shapeshifting is a common theme in myth, often related to journeys through the spirit world. One has to wonder if it's a result of our essential helplessness. As strong or fast as a human can be, we always need gadgets like clothes and spears to survive in the wilderness. Animals don’t. Since we can’t bring clothes or guns to the spirit world, we have to take a form where we can defend ourselves. This would explain why humans most often change into predators such as eagles, tigers, or wolves.
I have a more optimistic spin, of course. By taking the shape of an animal in our dreams or spirit-journeys, humans experience reality from a different perspective. The world would be a lot better if everyone could take a break from being human every once in a while.
There is an interesting twist that complicates this, though: many cultures believed that animals could turn into humans. That's a legend for another day, though.