The giant turtle Genbu is one of four spiritual beings with roots in Chinese mythology. Genbu is the creature's name in Japanese. In Chinese, he is known as Gui Xian, Kuei Hsien or Zuan-Wu. Genbu corresponds with winter, cold, water, black, earth and faith. The turtle is also a symbol of long life and happiness and was said to predict the future.
A turtle's shell also symbolizes a suit of armor, which is why Genbu is also referred to as the Black Warrior
Source: Japanese Buddhist Statuary