According to the Chinese astrology calendar, 2011 is a Chinese Rabbit Year. The Chinese calendar combines lunar, solar and Stem-Branch cycle systems. Stem cycle is the Yin Yang Five Elements. The Branch cycle is Rat, Cow, Tiger, Rabbit, Dragon, Snake, Horse, Sheep, Monkey, Chicken, Dog and Pig. Lunar and solar systems have been modified many times. The Stem-Branch system has not been changed for more than 5000 years. Rabbit is the 4th animal in the system. 2011 is Metal Rabbit, which is the 28th Stem-Branch in the cycle of 60 counting system. Chinese Horoscope uses animal names to build people birth charts for fortune-telling. Therefore Rabbit exists in the Chinese Horoscope system more than thousands of years.
The Vietnamese horoscope has 12 animals as well. They are Rat, Cow, Tiger, Cat, Dragon, Snake, Horse, Sheep, Monkey, Chicken, Dog and Pig. Rabbit is replaced by the Cat. Pets in Vietnam have been popular for thousands of years. Vietnamese keep dogs to guard their houses and cats to protect food and rice from rats. The cat is a smooth talker, talented with a patient personality and stays in conflict with the rat. The cat hours are between 5 a.m. and 7 a.m. – when cats begin to prowl. Cats are closer to humans than rabbits. Therefore, Vietnamese prefer cats over rabbits as zodiac animals.
It's true that Rats are afraid of Cats. But we cannot see this relationship from the Chinese astrology theory. Rat is in the Water group and Rabbit is in the Wood group. Cat is supposed to in the Wood group, too. Water is the mother element of Wood. Rat and Rabbit don't have a direct conflict. Rat and Cat should do the same. Then mother Rat shouldn't be afraid of child Cat.
Rat is active in the dark during the midnight. Therefore, 13:00 p.m. to 1:00 a.m. is the Rat hour. If cat sleeps during that time, cat cannot guard the food. Cat then wouldn't see the rat after 5 a.m.
Tiger and Cat are felids, which is the biological family of cats. Since they are similar, it is not suitable to have them both in the horoscope.
Originally, there were no animal names in the early Chinese Calendar. So why do the Vietnamese have cats instead of rabbits in their horoscope? Vietnam is next to China, and their culture has been influenced by China since thousand years ago. We can assume Vietnamese learned Chinese astrology from China. The Chinese characters (symbols) for the Stem-Branch cycle in the Chinese astrology are invented from the four-season Yin Yang natural phenomenon. Between 5 a.m. and 7 a.m., seeds sprout from the ground. Chinese saw one tiny stem with two leaves, and so created a pictographic character for this sign. Today it transforms into , which is pronounced Mao. The sound of Mao is same to today's Chinese calendar of , which means "emitting". That's true meaning of Mao.
Mao is the symbol for Rabbit in Chinese Horoscope. Rabbit in Chinese is , which is pronounced Tu. Cat's Chinese is , which is pronounced Mau. It is possible that Chinese people talked about Mao but as the word spread from mouths to ears, it might eventually be heard as Mau to Vietnamese. Then becomes and Rabbit becomes Cat.