Before Snow White, before Silly Symphonies and before Mickey, there was Oswald The Lucky Rabbit.
Originally created by Walt Dinsey and Ub Iwerks, Oswald was the first animated character created by the team in 1927. At the time the animated shorts were distrubuted by Universal studios but trouble began to brew when Disney requested an increase of budget from produce Charles Mintz to improve the cartoons, this unfortunately was counteract by Mintz desire to make a 20% budget cut.
Walt, Iwerks and a handful of other employees who had remained loyal left Universal, resulting in Walt disasscoiating himself from Oswald due to ownership of the character still being under Mintz as he was the one who commisioned the shorts originally.
In their new their new studio, Disney and Iwerks created Mickey Mouse, whom would become disney's most recognisable character. It's clear when looking at the two they were created by the same people and in many ways you could say that Mickey is just a variation of Oswald, luckily it was enough of a variation to stop any accounts of plagerism.
Over the years the strength on the Walt Disney Company has grown to an incredible size resulting in 2006's re-aquistion of rights from rival entertainment companies such as NBC Universal. With in this aquisition were the rights to Oswald Rabbit and his likeness. This deal was made via an agreement between the two companies, the rights to Oswald were traded with the contract for a sports presenter.
Whilst The Walt Dinsey Company have the full rights to all the original work (all epsiodes up until Disney's departure from universal) and Oswald's likeness, Universal still retains the rights of the animated shorts that were made without Disney, resulting in Oswald appearing in Disney media and merchandising but also previously made animted shorts from universal such as Woody Woodpecker and Friends. Most recently The Walt Disney Company exercised their right's of Oswalds ownership in their latest Video Game Epic Mickey, where in Oswald plays a large part to the narrative of the game.
Whilst Disney and Iwerks had created Oswald they did not actually own the rights to him, the studio did. LOuckily this resulted in the birth of mickey and The Disney Company taking back the rights to Owald in 2006, the most important that should be taken away from this case is that copyright can often be a tricky mistress in terms of creative ownership. When creating a character or being commisioned for work as a creative you should always read any contract you're going to sign concerning any hired work and that you should always be prepared for the worst consequences should something such as ownership rights pop-up.
Luckily stuff like this can usually be avoided fairly easily if the right precautions are taken and since 1927 copyright law has developed to help in settling disputes such as the one that took place between Mintz and Disney.