I did a costume class with my horses in 4H. Two of my favorites were fireman/firetruck or we also did a Beach/Hawaiian luau theme. For the fireman, I just wore a kid's toy fire helmet and one of those yellow plastic rain suits with the pants and jacket. My mare had a red sheet over her saddle and I just decorated it to look like a truck and I carried a length of hose. Pretty cute. Another year, my gelding and I did the Hawaiian luau. I had a ton of fake flower leis. I made a big one for him around his neck, put some in his mane and tail, and around his front legs. I wore a Hawaiian shirt and just threw a fake grass skirt over my jeans. My boy is a palomino, so I draped a colorful beach towel over the saddle so it looked like I was sitting on the sand!
Hope you have a blast. Costumed classes are always fun!