Mickey was a replacement for Oswald
Every big Disney fan I’m sure already knows this one! Oswald the Lucky Rabbit was Walt Disney’s first big creation, in 1927. It wasn’t until 1928 that Walt discovered that Universal Pictures, who his studios were then operating under, owned a lot more of Oswald than he first realised. They had taken on most of his staff under their own contract and retained the rights to Oswald. Walt Disney and a loyal animator, Ub Iwerks, refused to continue working under Universal and created their own replacement for Oswald.