Additionally, the coloration of this particular "bad guy" should always be something dark. Because the coloration of an animal is a clear indication of its character which, in this case, should be inexplicably soulless. Have you ever heard the saying, "never let a white cat cross your path?"
...of course you haven't. Because white cats simply don't do that shit.
2. One is All Does your evil animal belong to a particular species? If so, make sure that every single member of that species has something wrong with them. In fact, sometimes the species of the animal should dictate its moral standing. Ex. All rats are bad, all mice are good. Look at how well it worked out for Redwall. It's so much easier to
However, there -can- be some exceptions. Sometimes a member or two of a "bad" animal species can have a shot as a good guy. You just have to dumb them down far more than their evil and "cunning" counterparts and give them at least a few lines of whimsical singing dialogue. Making them appear overtly buffoonishwhile removing all traces of inherent species badassery certainly helps this portrayal along.
On the other hand, some have found these roles a bit "stale" in the past and, to spice it up, at times it helps to have the animal villain reform or temporarily switch sides/turn to good....so long as you make sure they meet a horrendous death before the story is over
3. Mortal Enemy Every villain needs an opposition, and animals are no exception. However, in this case it helps to make the rival an animal too. It can really help to make the "good" guy a natural rival of your antagonist. Ex. if the bad guy is a cat, make it a mouse, if it's a snake, make the opposite a mongoose, etc. etc. If you don't feel like doing that, make the "good" animal some sort of non-ugly, non-gross animal. Points for fluffiness can and will be hugely awarded (as will light coloration to indicate purity and/or goodness).
4. Meat, Shmeat Nearly, if not all meat-eating animals are evil/bad/tyrannical monsters that exist solely to terrorize the peaceable herbivores. Self-explanatory.
Likewise, animals that are often victims of this include but are not limited to: mice, deer, rabbits, squirrels, chipmunks.
Animals that are evil or often exhibiting questionable moral traits include but are not limited to: Big cats, certain species of dog, birds of prey, and pretty much every reptile ever.
Foxes and certain species of cat can fit neatly into either of these two categories but a number of times they can be seen walking the 'neutral' line as well.
5. Vermin Like their human counterparts, animal villains need a small force of their own to control. These "vermin" forces can be composed of the same species, or a species that is lesser/smaller than the villain him or herself, but as long as they're all exceedingly stupid it really shouldn't matter all that much. That, and the leader is incredibly smooth-talking and smooth-walking while having
one of the most memorable songs/monologues in the film/story, series, etc.