Introduction:
There are 12 animals in the Chinese zodiac: Rat, Ox, Tiger, Hare, Dragon, Snake, Horse, Sheep, Monkey, Rooster, Dog and Pig. And there are several theories on why those animals and not others are included.

Version 1: Buddha Send-off
One version says that these were the ones that went to bid Buddha goodbye and their reward was inclusion in the Zodiac.

Version 2: The Race
The Supreme Being, the Yellow Emperor of Heaven, Huangdi, decided one day that humans needed some kind of compass to guide them through the invisible maze of life's undertakings. So he decided to create the zodiac. To decide upon the symbols he announced to all the animals of the world that there would be a race, and that the first twelve beasts to cross the finishing line would be chosen to stand forever as signs of human character and destiny.
On the eve of the great race all the beasts went to sleep early to be fresh for the dawn start. But in the night Ox suddenly awoke and though: "I am so slow that if I am to have any chance at all of being among the twelve I had better start now." So he did, but as he passed among the other sleeping animals he woke Rat, who guessed his plan and thought : "I am so small that no matter how hard I run even Ox will beat me at this rate." So he scurried up a tree and dropped so lightly onto Ox's back as he passed below that he did not notice.
At dawn the other animals awoke (all except Dragon, who overslept), found that Ox and Rat had gone and raced off after them as fast as they could, but no matter how fast they ran they could not catch up. When the finishing line came into view they all saw Ox comfortable ahead of them and about to cross the line. Ox paused to look back and was pleased to see how well his plan had worked. But as he paused, Rat leaped off his back and raced across the line. Which is why Rat comes in first in the zodiac, followed by Ox and then all the rest of the twelve.

Version 3: Buddha
Buddha called all the creatures to him, but only the 12 made it.

• The Chinese people wanted a way to remember their history so they petitioned the Jade Emperor for advice. He same up with the race concept.
• The race course was to cross a swift river.
• Cat and Rat conspired and asked Ox to carry them across. Part way, Rat pushed Cat off into the river, then once Ox reached the other side, Rat jumped off and crossed the line first.
• Dragon stopped to provide rain in another area and then created a breeze to help Rabbit float across.
• Cat didn't make it in time, which is why he is always chasing Rat.