Kurma, the tortoise avatar of Vishnu, appeared in the Satya Yuga. In this incarnation, Vishnu took the form of a tortoise to support Mount Mandara during the churning of the cosmic ocean (Samudra Manthan), allowing the devas and asuras to obtain the nectar of immortality. Matsya, Kurma, and Varaha are all Satya Yuga avatars.