The mouse, or rat, is the first in the cycle of 12 animals representing years. The others are, chronologically, the ox, the tiger, the rabbit, the dragon, the snake, the horse, the goat, the monkey, the rooster, the dog and the pig.
How come animals designate years? Why 12, no more, no less? And why these specific animals?
There are different explanations about their origin. A popular legend says, long time ago, king of the heavens ordered all the animals to pay him a visit on New Year's Day, that is, the first day of the first lunar month. He said he would give the first 12 animals the title "King of the Animal World" and let each hold the title for one year. The 12 winners happened to be those mentioned above.
Another theory holds that the animals originated from the 28 constellations, or the Lunar Mansions, which are named after animals. Every two or three constellations stand for a year, and the most commonly known animal in each group was chosen for that year. Thus we have the 12 animals.
A more convincing theory maintains that using animals to symbolize years began from totems of minority peoples in ancient times. Different tribes had different animals as their totems. Gradually, these animals were used as a means to remember the years.
Alongside the increasing exchanges between the hinterland and the border regions, the custom of using animals to designate years made its way to the hinterland and was adopted by the Han people, the largest national group in China.
At that time, the Hans were using the 10 Heavenly Stems and the 12 Earthly Branches to designate years. They took one from each series to make a pair for one year and developed a system based on a 60-year cycle Every 60 years it is back to square one and the cycle repeats.
When the method of using animals to represent years was introduced into the hinterland, the ancient Chinese married them to the 12 Earthly Branches, one to each. So 12 animals were used. And animals officially began to be used to designate years during the Later Han of the Five Dynasties Period a little more than 1,000 years ago.
The New Year visit-to-the-god story explains how the unpleasant Mouse managed to become the first of the 12. As the story goes, when the Ox heard of the God's decree, he said to himself: "It's a long journey to visit that God. I am not a fast traveller and I'd better start early." So he set out on the eve of the Lunar New Year. The Mouse heard the Ox and jumped onto his back, without being noticed. The Ox, sweating all over, was so glad to be he first to arrive at the God's place. But just as he was about to be the first to arrive at the God's place, to express his New Year greetings to the God, the Mouse jumped down over the Ox's head and became the first to bow to the God. So he was appointed the first King of the Animals and consequently, the first of the 12 animals to designate years.
A more authentic explanation says, the Earthly Branches are divided into two categories: Yin and Yang. Each of them is paired with an animal of the same "gender". The gender of the animal is determined by the number of a specific part of its body. Odd numbers are Yang and even numbers are Yin. The tiger, the dragon, the monkey and the dog, all have five toes on each foot or paw, and the horse has one hoof. So we know they are Yang animals. The cloven-hoofed species such as the ox, the goat and the pig fall into the Yin category because their hoofs are divided into two parts. The rooster is also Yin since it has four toes on each foot. The rabbit has two upper lips and the snake has a two-point forked tongue. So they are Yin, too. The mouse had been a problem. It has four toes on each fore leg and five on each hind leg. It has both Yin and Yang qualities and there seemed to be no pace to put it. Fortunately, the first of the Earthly Branches, Zi (?)can be considered both Yin and Yang. The branches were also used to designate days and hours, and when symbolizing the hours, this branch covers a period from eleven in the evening to one o'clock in the morning. PM is Yin and Am is Yang. So the mouse goes together with this first branch. It is this double-gender feature, a kind of split personality, you might say, that makes the little mouse the leader among his colleagues.
Now you may be wondering why there is no Year of the Cat, especially since cats have been popular as pets for thousands of years in China as well as in many other countries.
Well, in the legend, the Cat failed to be chosen because he was a day late getting to the God's place. The Mouse had played a trick on him. He lied to the Cat, telling him the wrong date for the competition. The cat was not pleased and has hated the Mouse ever since.