from Wikipedia: Trolley Troubles is a 1927 animated short subject film, produced and directed by Walt Disney. The cartoon is noted for being the first appearance of Oswald the Lucky Rabbit, a character that Disney and Ub Iwerks created for Universal Pictures and Charles B. Mintz. Oswald would later serve as the basis for Mickey Mouse.
In the early summer of 1927 Disney finished the first Oswald cartoon. It was named "Poor Papa" but Universal was not very satisfied. They had expected a more Charlie Chaplin-like character and thought Oswald was too big and too weak. Disney agreed to make some changes and the cartoon was not released in theatres at the time.
Instead, Oswald's second-produced cartoon was submitted for release: "Trolley Troubles". Universal was pleased and the short was released by Universal on September 5, 1927. In the cartoon, Oswald is a trolley conductor. Due to road trouble, however, the trolley car goes completely out of control.
The press loved the new cartoon series Walt made and Oswald became a true hero. From that moment on, a new cartoon was released every two weeks.