Lord Sri Maha Vishnu incarnates on Earth from time to time to eradicate evil forces, to restore the dharma. He has taken nine avatars till now and the tenth Avathar will descend on Earth at the end of the Kali Yuga. These ten incarnations of Lord Vishnu is collectively known as the Dashavatar. Out of his ten Avatars he chose the form of animals in four of them.
The first four avatars of God Vishnu appeared in Satya or Krita Yuga (also known as the Golden Age). The next three appeared in Treta Yuga, the eighth and ninth in Dwapara Yuga and the tenth (Kalki Avatar) will appear at the end of Kali Yuga.