According to Chinese calender each year is a year of specific animal.There are 12 animals which are as follows.
1) Rat in Chinese shu
2) Ox in Chinese niu
3) Tiger in Chinese hu
4) Hare in Chinese tu
5) Dragon in Chinese long
6) Snake in Chinese she
7) Horse in Chinese ma
8) Sheep in Chinese yany
9) Monkey in Chinese hou
10) Rooster in Chinese ji
11) Dog in Chinese gou
12) Pig in Chinese Zhu.
2006 was year of Dog & 2007 was year of Pig according to Chinese Astrology .Therefore 2008 is year of Rat.Thus each animal appears after 12 years.