The Rat is the first animal in the 12 year cycle.
When the Jade Emperor held a contest, a race to be the first to reach his throne, the rat knew he didn’t have a chance. There was a big river to cross and the rat was a horrible swimmer. The rat was however very intelligent and knew the Ox was kind and a good swimmer, so he hitched a ride from the Ox. Just as the Ox was reaching shore the clever rat scrambled over the head of the Ox and jumped to land before the ox could step foot on shore.