It all began in the spring of 1928, when Disney asked Ub Iwerks to start drawing up new character ideas as a replacement for Oswald the Lucky Rabbit, an earlier cartoon created by the Disney Studio. Iwerks tried sketches of various animals, such as dogs and cats, but none of these appealed to Disney. Walt Disney got the inspiration for Mickey Mouse from his old pet mouse he used to have on his farm. In 1925, Hugh Harman drew some sketches of mice around a photograph of Walt Disney. These inspired Ub Iwerks to create a new mouse character for Disney. Ironically, "Mortimer Mouse" had been Disney's original name for the character before his wife convinced him to change it, and ultimately Mickey Mouse came to be.
The Minnie Mouse character was also created at the same time as Mickey. Mickey and Minnie debuted in the cartoon show Plane Crazy, first released on May 15, 1928. The cartoon was co-directed by Walt Disney and Ub Iwerks. The plot of Plane Crazy was fairly simple. Mickey is apparently trying to become an aviator. After building his own aircraft, he proceeds to ask Minnie to join him for its first flight, during which he repeatedly and unsuccessfully attempts to kiss her, eventually resorting to force. Minnie then parachutes out of the plane. While distracted by her, Mickey loses control of the plane. As a result, the aircraft eventually crash-landed.
In a nutshell, Mickey Mouse has indeed become a worldwide-known national phenomenon. Despite the fact that it being a pet mouse, it is an