The best costume I've ever seen was at the Newtown Bridle Trails hunter pace. The two women were a "bubble bath."
they took shower curtains for quarter sheets and covered themselves with semi blown up white balloons. toilet brushes for a jumping bat. shower caps over their helmets. it honestly was the best I've ever seen. I'm sorry I don't have a picture of all the details but it really looked great and was moveable.
The last time I did a costume class, I dressed up as a witch and my horse was the "broom". I wore black turtle neck with black breeches, black paddock boots and 1/2 chaps. I made a cape out of a piece of black cotton and black netting which I ripped into long strips. I left a couple of inches at one end of the material sort of as the collar and then took a weird looking silver pin to keep it around my neck. I got a black wig and witches hat, black lipstick etc for makeup. I took an old timey type broom and tied it to the side of my horse. My horse had black and orange streamers in her mane and forlock. Over her butt, I tied some of the fake cobweb stuff to the back of my saddle. To that I tied spiders. The only problem with the costume was the handle of the brrom was so long we couldn't turn to the right because it kept hitting my mare in the neck.
I haven't done a costume class in years, but here are a few of my favorites from back in the day:
(1) The Goodyear Blimp (my friend's pony was really, really fat)
(2) Bedouin - I leased an Arabian for a while, and made him a set of robes, and the traditional head covering. In retrospect, it would have been hilarious to make him the camel.
(3) A Chinese Dragon (a la Chinese New Year)
(4) Don Quixote