Mickey Mouse by Walt Disney and Ub Iwerks, ©1928
Mickey Mouse has arguably been the driving force behind the ongoing extension of copyright over the past 40 years. He was created in 1928, and it was the impending expiration of the Mickey Mouse copyright (among others) that prompted Disney (and others) to start pushing Congress for longer terms. They got it for Mickey’s 50th birthday.
Ironically, they never really needed it. Mickey as a character is practically a non-entity. He has no personality, and no memorable stories about him (the closest being his appearance in Fantasia, where’s essentially an actor playing the role of the Sorceror’s Apprentice). Although Disney is making some moves lately to (re)establish him as a character, pretty much the only purpose he serves for Disney is as a trademark for the company. And you don’t need copyright for that.
If Mickey had entered the Public Domain at the end of 1984 like he was supposed to, it would have allowed other people to use the character in their own cartoons, comics, etc. But they wouldn’t be able to use the name “Mickey Mouse” (a trademark of the Walt Disney Company) anywhere on the packaging or promotional materials for it, and they wouldn’t be able to use the distinctive likeness of the character (also a registered trademark) for that either. They’d effectively have to keep his presence in the story a secret, or risk the legal might of one of the most powerful media conglomerates ever.
So in the same sense, Mickey is really more a symbol of something larger: characters owned by corporations and other heirs of their creators, who represent potential property for the holders of their copyrights.
I think you’re wrong about people not being able to call Mickey Mouse by his well-recognized name. Corporations have a choice between registering copyrights or trademarks or patents. They don’t get to assert all of them, or more than one, all at once. The initial claim of intellectual property is in the copyright for Mickey Mouse, not a trademark. If he’d been invented and trademarked all at once, we’d all be screwed. But he wasn’t. They pursued some claims for particular examples of his image and possibly particular examples of his name designed in certain particular ways later on.
Mickey Mouse of course is probably already in the public domain and always has been because of the well-known imperfections in the initial copyright claim. But even granting Disney the current copyright term that supposedly adheres to Mickey Mouse, when he’s in the public domain, he’s in the public domain and he can be called by his name.
If they’ve trademarked terms like “Mickey Mouse Socks” or “Mickey Mouse Cowabunga Adventure Ride,” then, no, you can’t use those particular terms, his name was his name from early on, and I don’t think they tried to do any trademarks with it until the mid-30s, after the fact.
You should get some expert analysis by people like the lawyers who have determined that he’s in the public domain already.
Anyone who says that Mickey Mouse is in the public domain is – by definition – not an expert.