We all have a spirt animal. One of my best friends is most definitely a camel. Somedays he acts more like a slug or a manatee. Other times he can stand tall like a stag or soar like an eagle. But on average, his essence is that of a camel. It's not just the way he curls his lips to take an infrequent sip of water. It's not even that he doggedly dresses in khaki, beige, and tan. It is that at his core he has the character of a camel - enduring, awkwardly graceful, rarely affectionate, fearless, and occasionally cranky.
Let us say (with the possible exception of dolphins, chimps, and some other mammals) that animals posses basically the same brain architecture and chemistry as humans, without the highly developed neocortex - the area of the brain that gives rise to things like culture, morality and philosophy. Without the influence of higher order thinking, animals go through life largely on instinct, impulse, and the necessary motivations for the continuation of life.
But just as evolution has given rise to the amazing diversity of species on our planet, it has given rise to an equally diverse collections of behaviors between the species. When seen through human eyes, these behaviors seem to make up the basis for a personality. Although fire ants, honey bees, and praying mantises are all insects, they have vastly different "personalities." The more you learn about an animal's behavior, the more interesting and complete their specific personality comes.
Animals are eternally captivating because they play out our primal instincts fantastically - with claws and wings and teeth and resplendent beauty and outrageous ugliness - but without the burden of morality. Additionally, animal personality are extremely diverse. I challenge you to find a human personality that can't be modeled with an animal.
Brands tend to simplify the complexity of animal behaviors so that the logo embodies the most captivating quality. This is still quite effective as animals, like archetypes, are direct, primal, widely relevant, and timeless.