In the standard Chinese zodiac order, the Rat is the first sign.
The twelve-animal sequence begins Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog, and Pig. This order is used for years, although the exact start of a zodiac year follows the Chinese lunisolar calendar rather than January 1.
A famous legend says the Rat won a race arranged by the Jade Emperor. It reached the finish first after asking the Ox for help crossing a river, then jumping ahead just before the finish. The story explains the order symbolically, not historically.
The Rat is also associated with the earthly branch Zi and, in traditional timekeeping, the period from 11 p.m. to 1 a.m. The branch association and the animal order are related parts of the same calendrical system.