Well, I read your post when you posted it, but had to think about some ideas.
I've always been partial to the indian horse and indian rider, and the Arab styles (which I think this one could be more difficult). But I googled horse costume image and saw alot of ideas on there that may give you an idea that you like. My mind was picturing Dr. DooLittle with a "push me pull me" the 2 headed Llama, and I pictured a 2 headed horse, if you could figure out how to do it would be cute. I also pictured something like cupid on a horse, rider dressed in red with a gold wrap, and a red bow with a quiver of red arrows and a red arrow in hand with a heart for the tip. The horse having a white irredescent cloth drapped over it's back, a white face mask (if obtainable), decorate the mane and tail real pretty in gold, red and white irredescent ribbons, and could do something cute on the back end, like a red bulls eye painted on.
I think it would be so fun to create a costume and be in a costume class or in a parade! :~) I hope that when you do it, you'll share some pictures! I'd love to see some. :~)
~Karen