The idea of a spirit animal comes from a Native American tradition, though it is not common to all Native American faiths and cultures (as there are many, and they are far from homogeneous). It has appeared throughout mythology and even pops up today in pop culture, for example in the "daemon" that is the manifestation of a person's soul in Phillip Pullman's "His Dark Materials" trilogy. Indeed, the idea of a Patronus in Harry Potter - a protective force that appears as an animal and correlates to a person's inner character - is a direct nod to the concept of the spirit animal.
How do you know what your spirit animal is? It's not just an animal that you like, or with which you feel some kinship because it has a gesture or a feature that looks like you. It goes much deeper than that. A spirit animal is a creature who faces the world the same way you do: who is social or antisocial, fearful or bold, playful or serene just as you are. It's the animal species whose Platonic ideal would be most likely to choose the way you do in any given situation. So let's find out which of the many souls in the animal kingdom has managed to sneak just a little of itself into your body!