Sheep is one of Chinese favourite animals.  Sheep's Chinese character is 羊 (Yang).  The Chinese character of Luck 祥 contains the 羊.  The pronounciation of 祥 is Xiang, which is close of Yang 羊.  Therefore, Sheep becomes a lucky animal.  The sound of 羊 Yang is the same as 阳 Yang of Yin Yang.  阳 Yang means the positive energy in the universe.