The Rat and the Cat
A long, long time ago, the Cat and the Rat were the best of friends. They accompanied each other everywhere they went and they shared their food. One day, the Cat learnt that the Jade Emperor was going to elect twelve animals to represent the twelve calendar years, and invited all animals to a party. The Cat suggested to the Rat that both of them should make a presence.
When the day finally arrived, the Cat told the Rat, 'I am going to take a nap so that I will be in my top form. Can you please wake me up when it's time to go to the party?' 'No problem.' the Rat replied, 'Just relax and rest. I will wake you up when the time comes.'
After the Cat fell asleep, the Rat thought to himself, 'I am among the smallest of all animals and I don't stand a chance in making one of the twelve if the Cat will go.' So when the time came, the Rat left on his own without waking up the Cat, and was elected as one of the twelve Zodiac Animals.
When the Cat found out what the Rat had done, he was furious. The two became worst enemies. This is the reason why cats are not one of the twelve animals and they love to chase after rats.
The Rat and the Bull
There are numerous stories on why rats (despite their small size) are ranked at the top of the zodiac chart. Here is one version.
After selecting the twelve animals, the Jade Emperor asked them to rank themselves from the greatest to the smallest. A long debate was resulted and when all the dusts settled, the Bull* was ready to claim the top spot, but the Rat objected. 'I should be at the top of the chart because I am the most feared animal in the jungle.' The Bull began to laugh in despise, and the Rat continued, 'The best way to illustrate this, is for me to travel through the jungle, so that you would see the feared look on their faces. However, it will take me a long time to travel through the jungle. Do you mind giving me a ride?' The Bull agreed and the two proceeded towards the jungle.
When the animals saw the Bull and his angry look, they were dead scared and started to run. None of them saw the Rat, which was sitting between the horns of the Bull. But the Bull mistakenly thought that the animals were running away from the Rat, gave up his top spot unwillingly, and assumed second space on the zodiac chart.
*It is not surprising that the Bull was ranked above the Tiger and the Dragon. Fights between bulls and jaguars are very common in South America; the felines don't stand a chance against the fierce bulls. The Dragon was probably a large lizard.
The Rat and the Elephant
This is another version why the Rat made it to the top of the zodiac chart. To convince their piers why they deserved to be at the top of the zodiac chart, each animal gave an account of its merit.
The Bull said it helped the farmer to plow the fields.
The Tiger said it was strong and powerful.
The Rabbit said it was the famous Jade Rabbit of Chang Er.
The Dragon said it symbolized Kingship and Royal Descendant.
The Snake said it was smart.
The Horse said it could travel thousands of miles.
The Lamb said it provided wool for clothing.
The Monkey said monkey-see-monkey-do, and it could do whatever human could.
The Rooster said its calling woke up the farmer in the morning.
The Dog said it was the best friend of human and it protected them.
The Pig said its meat tasted good.
When it came to the Rat, it simply said, 'I am the king of the jungle!' Everyone laughed but the Rat continued, 'Who do you think was the king of the jungle?' When all the animals agreed that the Elephant was, the Rat swiftly ran into the ear canal of the Elephant and brought it to its knees. After this, all animals agreed that the Rat should make it at the top of the list. Rats!