It's depending. Putting clothes on cats is pretty much impossible, but people do it too dogs constantly. I don't really like seeing doing is costumes, cause it seems a little mean, but clothing for dogs can be a good thing! For instance, my friend has a tiny yorkie. If she didn't put it's little coat on in the winter, the poor thing would probably freeze to death! It was one of those Velcro wrap around things, where these two holes for the front legs and the rest just sort of drapes over it. It would probably look like a sort of a cape if it were for humans, haha. I also know people who will have the "shirt" type clothing for their dogs; it's usually thin material and says something cute or funny on it or is done with a neat patterned fabric. They usually soak the shirt is water and the wring it out and place it on the dog to help them keep cool.
Costumes for pets is very silly to me, but all in all dog clothing can be a very good thing.
