Our dog has the superman costume but will only wear the cape (he lays down on the floor if I put the body on him) and he is very superhero'ish in it. All our trick or treaters loved him!
And, mudbug, our cat has a princess hat (think old school princesses, not tiaras) / cape and a witch hat / cape that she will wear around the house for awhile. I think she likes the capes, she curls up to sleep and can tuck her head under the end to "hide". The hats, not so much.