The four mythological creatures of China are the symbols of four directions. They came from ancient Chinese Astronomy. After organized the culture relics and the literature, this study points out: the creature of the north was a deer before the West-Han Dynasty. This creature came from the 2 mansions in the north part of the 28 mansions. Due to the influence of politics and superstitions, it became to another creature combined by a turtle and a snake, which came from another 2 mansions in the sky at the early period of the West-Han Dynasty.