One year I painted him to look like a zebra, one year we were Ringwraiths (from Lord of the Rings).
This year I'm painting orange stripes on him and he's going to be a tiger and I'm going to be an Indiana Jones type lion tamer.
Other ideas are:
cartoon characters (little mermaid is one that is cool to see)
famous duos (bonny and clyde, batman and robin, jack sparrow and wil turner)
themes - (medieval- space-western-oriental aka crouching tiger hidden dragon- country-)
I have a lot of ideas actually but it really helps to know what the horse looks like.
how about "Ant on a picinc blanket" "Knight on its horse" "Cowgirl on a western horse" " lifegaurd on a blanket" " a sheep hearder and a sheep" " Angel on a horse with wings" " Little red riding hood and the big bad wolf" " bumble bees" " An elf with a reindeer" "Chefs" " McDonalds fast food" http://www.thehorsetailor.com/Costumes.html
This wedsite shows some of the things that i said. I hope this help!
I did one one year where I put a straw hat on my horse with his ears poking out and then I wore overalls and pig tails. I had a red bandana on and then I had a stick with a carrot tied to a string, I rode bareback with rope for reigns and held the carrot infront of him to make him go... it was pretty cute....
:) There are some really clever ideas in response to your costume request.
What came to my mind was a farmer on a tractor... with of course the horse being disguised as a tractor with "wheels" where your stirrups are.
It seems like poetic justice to me.