Something I struggle with when I start any new writing project is what species to make each of my characters. Usually I can come up with the species based on personality, but then there are times where I wonder if I should conform to what I think an anthro fox, wolf, rabbit, rat, etc. would be like, or should I break the mold because does the bad guy always have to be a rat or a crocodile or does the wily one have to be a fox or a weasel?

Any other writers out there have any tips on how to come up with species for your characters?