According to one myth, the Jade Emperor said the order would be decided by the order in which they arrived to his party. The Rat tricked the Ox into giving him a ride. Then, just as they arrived at the finish line, Rat jumped down and landed ahead of Ox, becoming first.
The Rat is also associated with the Earthly Branch (地支—dì zhī) Zi (子) and the midnight hours. In the terms of yin and yang (阴阳—yīn yáng), the Rat is yang and represents the beginning of a new day.
In Chinese culture, rats were seen as a sign of wealth and surplus. Because of their reproduction rate, married couples also prayed to them for children, we wish you all a very happy and prosperous new year.