Twelve Chinese Zodiac Signs
The twelve Chinese zodiac signs are the twelve animals Chinese people customarily use to mark the year in which a person was born. The twelve animals and the order in which they are arranged are called "the twelve zodiac signs". The twelve animals are rat, ox, tiger, rabbit, dragon, snake, horse, goat, monkey, rooster, dog, and pig.
The formation of the twelve animal signs is based on astronomy and has something to do with the Chinese way of numbering the years according to Heavenly Stems and Earthly Branches. Marking a person's year of birth by combining numbers from the ten Heavenly Stems and the twelve Earthly Branches is no easy thing for most people.
In order to make things easier for calculation and remembering, a system of twelve animals like rat and ox, representing numbers in the Heavenly Stems and Earthly Branches was adopted, with one animal marking the zodiac sign of each year. This special way of marking a person's age is a unique phenomenon in Chinese folk culture.
As to why these particular twelve animals are chosen, there are lots of legends in Chinese folk culture. The relatively scientific account is that the decision was from the primitive worship of animals. Another opinion is that the selection and sequence of the twelve animals was determined by the activity time of each animal. For example, the rat has been chosen to represent midnight, because it becomes active at night.
The twelve animal signs are not unique to the Han people. Many ethnic minorities also use them to number years, but there may be some difference in the animals selected.