Fido may look adorable in his new superhero costume, but that cape won't necessarily keep him out of harm's way. Dressing up your pet for Halloween can be harmful. Costumes should not constrict the animal's movement, hearing or sight, or impede their ability to breathe, bark or meow. Also, it may be helpful to try on costumes before the big night. If your pet seems distressed or shows abnormal behavior, don't bother.