The Chinese Zodiak, why animals?

I´m wondering, why these animals? Some are not considered to have much value in the western hemisphere, like the rat, which is the animal of this new year. And by the way is a dragon an animal?
I myself is born in a rooster year in the monkey month of september. I don´t believe my life is guided by starsigns or chinese animals, but find it amusing to read horoscopes. Before I married MH a friend made a couple horoscope (western) for us and found out that it was a good match. Which I already knew of course!

/EH

An artpiece of chinese zodiak animals


And a kids version...