I wish I had pictures of some that I've seen over the years. My favorite at shows I judged last season was a golfer & his golf cart. They built a fiberglass golf cart shell and put on his pony and he was dressed in traditional golf garb. The pony was wearing a sign on his butt that said "Tee'd Off" It was really cute.
Saw another that used a blue sheet and covered in a shad catch net and hung shells, etc from it and the rider was in full dive gear. Horse even had on flippers & snorkel.
I (many, many years ago) did zookeeper & zebra with my black mare and painted her stripes with white liquid shoe polish. Looked awesome until the white stripes wouldn't wash out and I had to show my zebra in my regular performance classes