* dress your horse up as a angel and you put on a devil costume
* put a orange blanket on him and you wear a basket ball jersey
* wear a swim suit and put a towel on your horse and a wistile on your rains goggles on the horse work good to
* Another costume idea is a knight in shining armour. This is very easy get a stable sheet or blanket, put it on the horse put the saddle over it, get synthetic bridle that matches. Got to a halloween shop and buy a knights costume and get a fake sword.
When I worked at a stable during the summer we had our own costume contest. I had 2 graduation stuff (the cap and gown) I put the gown around the horse's neck and let it drape on her back, and then I put the cap on top of her head and held in it with bobby pins I believe. Then I dressed up the same way. We won that little contest. Everyone said it was a really cute idea.
LOL. I just remembered, On Ebay (someone posted this) they had the rainbow fluffy boots. You could dress him up as a rainbow dragon or monster of some sort. Glue (or sow) so rainbow sequins on to a rig, get some fluffy rainbow horse boots. Then you wear something else. Like er.. a leprechaun.
I dressed Luke up as Harry Potter (pipe cleaner glasses, harry potter scarf, painted on lightning bolt, red/gol ribbon in tail) and I was Ginny Weasley (red/gold tie, black pants, cloak, etc.) I held onto a wand while riding too.