What a wonderful idea!
Maybe if you draw more of them you could combine them into a real coloring book - not only giving kids the chance to be creative with horses, but also to educate them about them. I can already picture drawings with different facial expressions to teach them in what mood their horse is. Brilliant idea!
I drew a Horse Mandala book a couple of years ago ( http://www.amazon.com/Horse-Mandalas-Ma ... ala+horses ) and around the same time I bought a great coloring book that had drawings of famous horse paintings in it. I forgot it's name, but your drawing really reminds me of it. I can't wait to see more!