Nixon's Head appeared in the pilot, savage Fry for knocking his jar to the floor. He subsequently whistled up some cop bots and hounded Fry, Bender, and Leela until they escaped in the Planet Express Ship.
It is unclear how the 20th century leader's head found it's way alive to the 30th century, but they didn't call him tricky dick for nothing. In the world of the future Nixon's Head is Earth's president.