Skippy! and I won a costume contest against people with REAL knights outfits for their horses (im talking waving banners and metal armor) because I dressed him up as a Paint Horse (Since he is a Breeding Stock Paint) And I wrote "The Pain't that Ain't" on his butt (two words on one side of his tail, two on the other) It was very clever and thats why I won, lol!
As for your horse... Maybe Peter Pan? LOL Get him a little green hat with a red feather, green polo wraps, and ride in your PJ's as Wendy.. or something :p You can even use horse paint to write "Off to Never Neverland" on his booty so people will get it, lol Its pretty inexpensive to do, too :)
Or you can buy a blue sleazy hood and put the Superman Logo on the front of it, I did that once too and it was a HOOT! LOL!
Just a suggestion ^^