The popular story in China said Rat win the competition among the animals. it is just a story. I think another version is reliable. Ancient people divided 24 hours of a day into 12 pieces. The first piece is from 11pm-1am, we called the time of 'zi'. This is the active time for rat because they began to stolen food. So, rat is the first place. Ox like taking food at late night around 2-3am, so, Ox is the second one. So, that is the order.