Is the event indoors or outside? The real question is, what will the temperature be wherever the party is. If the Halloween party for dogs is outside in a warm climate or indoors in a cool climate, stay away from costumes that will cause Fido to overheat. Costumes made of felt are inexpensive but they don't breathe which could be hazardous to your pooch's health. When selecting a costume, only consider fabrics that breathe.
Select the right size for your dog. It's easy to understand why a costume that is too small would not be good but a costume that is too big also presents several problems. Imagine a costume that is so loose Fido's front leg comes out of the armhole and gets entangled inside the costume when he's running. Or what if it's so baggy that when she squats she urinates IN the costume?
Consider the temperament of your dog when selecting a costume. While my Golden Retriever wouldn't even wear booties to keep his paws from freezing in 50-below weather during a Minnesota winter, my Olde English Bulldog won't consider stepping out without her coat. So a Halloween costume for my golden will be a very simple black bow tie affixed to his collar. On the other hand my bulldog will go all out this Halloween. She'll be sporting the Zelda punk rock dog outfit from Costume Supercenter, complete with Mohawk and pleather vest!
Consider the temperament of other dogs at the party. While it is every pet owner's responsibility to control their own dog, all pet owners know that not every owner accepts that responsibility. So while it might be funny to create a costume for your dog that includes dog treats affixed to it, that might also invite other dogs to attack your dog. Choose or create your dog's costume wisely.
Keep an eye on your dog at the party at all times. Most dog costumes are cheaply made because they aren't intended to last very long. As dogs run and jump and play, pieces and parts of their costumes may fall to the ground. If your dog is the type that will eat anything, you don't want her swallowing bits of plastic and cloth. Otherwise she'll have the same tummy ache after the party that I did as a kid after eating my stash of candy all in one night!
Keeping these five simple tips in mind will lead to safe fun for both of you. Happy Halloween!