Homer was the mascot of the Atlanta Braves. He has a baseball shaped head, and looks a little like Mr. Met. Before having the baseball head however, Homer was the personification of the old "Screaming Warrior" logo the Braves used before dropping it in 1988.
Homer's full name is Homer the Brave. This is meant to sound like "home of the brave", the last words of the national anthem. Incidentally, "homer" is also the longtime nickname for a home run.