There is a competition at my riding stables every year. The thing is to dress you and your horse up in fancy dress and who every does the most creative and original thing wins. Last year I was partnered with a grey appaloosa and the same again this year. here is an example. I cant think of many ideas and would like as many options as possible (at least 5 to get the 10points).
here is an example, but you can use the ones i have used as an example.
1) Indian
2) Cowboy
(they are the only 2 i can think of!)

medieval knight
trojan horse
Crusader
Arab horse, (lawrance of Arabia style - watch the move)
a Ferrari Horse and Rider (because Ferrari's symbol is a prancing horse)

Oh there is so many! We have fun dress ups at gymkahnas every year for the younger kids. There have been some AMAZING ideas. pets question and answers,www.5d2d.com
I know these aren't all 'horsey' but they still all worked really well. I'll just list the ones that have won recently.

Headless horseman (they just pulled a jacket over their head, and dressed up scary)
Pegasus (this girl had amazing hand made wings that fit over her horse's wither)
Unicorn/fairy
Princess pony
Pajama party (The girl had made a little sleeping cap for her horse, put flannel over it's back, and had little eye-covers !)
Little bo-peep. (The horse was dressed up like a sheep, and the girl like little bo!)
Circus horse. (Alot of spandex and Lycra! All very sparkly with feathers on top of the horses head)
Egyptian horse (alot of fake cold chains and gold material. The girls were dressed up in togas with a LOT of eyeliner)

Those are the best ones I've seen , anyway! I hope I've helped!
And have fun!

horse owner/rider

Army people
Police people
Indian people
Cowboy people
Carnival people ( i like this the best, you could wear a top hat and really colourful clothes)
gypsy people
rodeo people

Hope this helps
I really think the carnival one would look amazing!

my head

These are all realllyyy good ideas! I think dressing up as Katie Price would be funny... balloons down your top etc ha! But in all seriousness, I like the other ideas. :)

I was so excited about answering that one, but I saw all the other answers, way better than mine!
But, I'll give it a try...

Knight or anything medieval is a good idea
Someone royal on a horse maybe?
British policemen are nice
If your horse is up to it, maybe something with a carriage?
Military men
Zorro!!!!
Seabiscuit
Circus themed
Just for the outfit (for you and the horse, I think it's really cute!):
http://www.youtube.com/watch?v=pS9nwnfqS…

Mongolian! It's like that:
http://www.youtube.com/watch?v=Qs2urV-uZ…

Hope I gave you something to think about!