I take it THAT'S never been explained, outside of Kurtzman or the Air Pirates anyway.
I mean, the mice wear pants. In Mickey's case red showy ones, with big symbols for his huge Disney Corporation balls.The horses and whatever Goofy or the Beagle Boys are, thank god, wear pants.
But. The chipmunks do not. Pluto does not. And you can barely understand what the ducks say. I'm getting the feeling they may have sneaked into this anthropomorphic status. Scrooge's money being behind it.