I would normally not dress up my horses because they are mostly all deaf, blind, jumpy or too old to ride about at night, but I did help one of my friends with her horse costume.

I brought some paint that was safe for horses/ponies. (I can't remember the brand, so don't ask.)

It took sometime but I painted some rainbows from her horse's back and made them look like they were wrapped around the legs. Thinking back now that took awhile to clean the paint of her horse's coat.