Want a really easy effective costume? It doesn't matter what colour or type of horse you have (one with some white fur would be good) This is a really simple costume. All you need is some old clothes you can spatter in paint and some caution tape and signs
If you want you can used a water-based, non toxic washable paint, or some kind of kids craft paint works on your horse for splatters
Whatever you use i would suggest you test it on a SMALL patch for 48 hours before to check for any reactions to your horse. If in any doubt don't use it!
NB. I take no responsibility if someting you use has a bad effect as above if unsure don't use it!