Mickey Mouse is a famous Disney cartoon character, who stars in his own TV show of the same name, created by Walt Disney. He has become world famous and is now recognized as the mascot of Disney. He has appeared a few times in Family Guy.
In "A Hero Sits Next Door", Peter said he'll help Joe get a job on the Happy-Go-Lucky Toy Factory softball team faster than you can say "copyright infringement". Just then, his head turned into a Mickey Mouse head.
In "Dial Meg for Murder", Peter used a Mickey Mouse phone to take business calls.
In "3 Acts of God", Peter stupidly believes that Mickey Mouse is real and thinks it's wildly coincidental that Mickey Mouse happened to be at Disney World the same day he visited there.
In "Turkey Guys", The J.C. Penney's Thanksgiving Day Parade used non-copyrighted characters for their floats such as "Ricky Rabbit".
In "Peter's Def Jam", Mickey Mouse eats some bad shrimp on a Disney cruise and hops across the deck of a Disney cruise ship clutching the seat of his shorts in a bout of intestinal distress, warning the other patrons not to eat the shrimp.
In "Nanny Goats", Mickey Mouse orders a hit on Jewish mouse, Fievel Mousekewitz as an act of anti-Semitic hatred. Peter later comments that copyright law says they need to have a reason for using someone else's character and in this case, the reason they did this was because Walt Disney hated the Jews.