It's a monkey, because it refers to the Monkey King. The Monkey King is very powerful. It is the Monkey King's deeds to make trouble in Heaven. the Monkey King takes monkeys as the prototype.
Zodiac is twelve kinds of animals that match the twelve branches in China, including rats, cows, tigers, rabbits, dragons, snakes, horses, sheep, monkeys, chickens, dogs and pigs. The origin of the zodiac is related to animal worship, and each zodiac has rich legends, which has become an image philosophy in folk culture, such as the zodiac in marriage and the year of the animal.