If your pet is all ready to trick-or-treat then we want to see pictures!
If you don't want to go out and buy a costume, you can always make one at home. Here are a couple tutorials...
A straw hat plus a bandana from the craft store makes a quick cowboy or girl. Just like with kids costumes, a sweatshirt + fake fur/feathers, along with painted spots or stripes make for a cute bug or animal.