The word' Cheng' comes from a senior official in ancient times, lower than a king and higher than a hundred officials, so it is called' Prime Minister'. "Rat" ranks first in the 12 zodiac, but he is below human beings. So, the answer to this riddle is mouse.

In addition, there are rats, cows, tigers, rabbits, dragons, snakes, horses, sheep, monkeys, chickens, dogs and pigs.