If you're crafty, maybe you could make the stick horses.. Or perhaps you could look up some homemade horse treat recipes online and sell them at the show. You could also do some type of raffle for a big prize. See if a local tack shop would donate something (even a gift card) for the cause. We had a big car show 2 years ago for Breast Cancer Awareness and we sold T-Shirts, charged admission (not sure if you can do that), gave away trophies for best in class, and raffled off several things such as car cleaning kits, gift cards, TVs, game consoles, ETC.
I'll try to think of some other ideas as the day goes on.. I'm usually pretty good with fundraisers.
“Out of suffering have emerged the strongest souls; the most massive characters are seared with scars.” -Kahlil Gibran