FUN! We have costume contest every october at our barn!
We have a lot of witches, pirates, hula dancers, ballarinas, can can dancers, angels, hippies, bride/groom, etc.
Lots of movie/book characters: Tinkerbelle/Peter Pan, Harry Potter/Hermione, Little Bo Peep and horse was the sheep, etc
Random and fun: (we had a bumblebee Squeakers! It was hysterical!) Pizza Delivery guy and the horse was a pizza, horse was a hot dog (big bun on both sides of the pony), farmer and the horse was a cow, rider was a knight and horse was a dragon, etc etc.
Hope that gives you some ideas!