Never Force Your Pet To Dress Up*
Most pets don't enjoy wearing a costume and become uncomfortable, unpredictable, and strange to other pets and animals when dressed up. Instead, if you'd like to help them get into the fun, consider a Halloween bandana for them. For some pets, when they've been desensitized and positively reinforced, they may like being dressed up. I don't mean tolerate, but actually enjoy it. In these cases, make sure the costume doesn't interfere with their ability to move, breathe, see, hear or bark.
*Berlin does not like being dressed up, as seen in the Wonder Woman picture on this page, which was taken at my husband's office. Unfortunately, his boss insisted that all the office dogs were put in costume for a photo. It bothers me greatly she was forced to do this for the internet's amusement. Don't be that person.