For those that have been consumed with curiosity...

The mystery is solved. I received this email today from Marc Brown.

Dear Perplexed,

This question comes up a lot. To clear up the mystery:

Arthur, Baby Kate, D.W. are aardvarks

Francine and Muffy are monkeys

Buster is a bunny

Brain is a bear

Binky is a bull dog

Prunella is a dog

Sue Ellen is a cat

Hope that helps!

Marc Brown Studios