Once upon a time we turned a pony into a Star Wars
Bantha With the addition of a wooley blanket and some paper mache horns. The kid was a Jedi.
We have also done costumes from various nursery rhymes, fisher boy, Indian, Eskimo and walrus is one I have seen. We had a 17 hand black Quarter horse turned into Mary's little lamb with an old sheet and about a million cotton balls the little girl was Mary. We have done GI Joe and the "Jungle". Horses have been painted with tempera paint mixed with a little dish soap.
Really the sky is the limit as long as the pony can see well, maneuver safely and is not afraid of it. When in doubt make the entry a lead in.