Long long time ago, the Jade Emperor, organized an animal race. He told all the animals that the first 12 animals to win could get a place in the Chinese Zodiac (十二生肖), and have a year named after them.
The cat and the mouse both asked the ox to wake them up early on the day of the race. That day, the ox tried to wake them up, but without success. So the kind ox carried them on his back and started running when the race began. The mouse woke up just when the ox was going to across the last river. He knew that he could never beat the cat. So he pushed the cat off the ox's back. When the ox reached the other side of the river, the mouse jumped off and ran to the finishing line, just before the ox.
So the 12-year cycle(循环) begins with the mouse. The ox comes after him. The tiger follows the ox. The rabbit, dragon, snake, horse, goat, monkey, rooster, dog and pig come after them. The cat didn't get a place in the Zodiac. That's why the cat hates the mouse and always wants to kill it.