Tathagata is the name that Buddha said people should use to refer to him while he is still alive. When somebody attains a Perfect Enlightenment, he calls himself as Tathagata, which means that he is the one who has opened his eye completely to the world.
A Tathagata appears in the world once about every 3000 years. His mission is to reveal the way of the world for all mankind.
People who have attained Supreme Enlightenment and named themselves Tathagata: