What color is your horse?
I've done several with my horse - he's all black.
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.