Here's an analogy I gave my students last week...
Imagine you're an alien and you land on Earth in front of a pet store. You go inside and you start meeting dogs. Some are big with a loud deep "WOOF", some are small with a quieter higher "ruff" and there's one little one that goes "meow". Some of them have big floppy ears, some of them have little floppy ears, and that little one has sharp pointed ears with tufts on the end. You think "That little meowing dog with the pointed tufted ears is an unusual dog!"
Then you go onto the rest of the pet store and find a whole bunch more small meowing things with pointed tufted ears, and you say "Oh... I see. That wasn't a funny dog, that was just the first cat I met!"
Pluto was the first Trans-Neptunian Object we met, and so we originally called it by our existing language ("planet"). But once we had a much better lay of the land, it became clear that it was just the first example of a quite different type of object.