No one is sure when the Chinese zodiac started, but the animals of the zodiac appeared in artifacts as early as the Warring States Period of Chinese history in 475 B.C. to 221 B.C. The zodiac animals were already popular during the Tang Dynasty in 618 A.D. to 907 A.D.
Scholars believe that the Chinese zodiac originated from early Chinese astronomy which tracked the 12-year orbit that Jupiter took around the Earth. Other scholars believe that nomadic tribes of ancient China developed the zodiac based on the animals that they hunted and gathered.
The Chinese zodiac is based on the story of how the Jade Emperor invited the animals to come for a banquet and the order in which the 12 animals came. The first five to arrive were the rat, ox, tiger, rabbit and dragon. The next five were the snake, horse, goat, monkey and chicken. The last two to arrive were the dog and the pig. The year 2015 is the year of the goat.