Not the planet(oid)—he got punished for just being too far away from home and never calling. No, the dog. He musta done something very, very bad...so bad that he's been demoted to a second class citizen. Come, walk with me for a while while we talk.
Now, Mickey Mouse is a mouse, as is Minnie. Donald, a duck. Ditto Daisy, Huey, Dewey, Louie, Scrooge McDuck. All animals in the Disney Universe, and all walking upright and wearing clothes.
Then we get to Goofy. He's a dog. Walks, talks, manipulates objects with his hands, capable of complex, if often flawed reasoning. And then there's Pluto. Also a dog, but wearing a collar, spends most of his time on all fours, barking.
In a world of anthropomorphized animals, Pluto is the only pet. He's unique. I can't imagine the natural forces in that Universe that would render only one animal, of all the others, incapable of Higher Functionality, and willing to accept a life of slavery. The only conclusion I can come to is that Pluto is being punished for something. Maybe he sniffed a little too close to the Private Reserve Cheese. Perhaps he tried to put the moves on Minnie. Hell, maybe he's serving time for chipmunk-slaughter.
But he did something, and I wanna know what.
(This is the kind of shit that floats into the head of a ridiculously bored adult when faced with watching the same three episodes of Mickey Mouse Clubhouse for six months straight.)