best costume I ever did was "the motorcycle sluts from hell" so we drew tattoos on the horses and ourselves, tied crushed beer cans to their tails & feet (if tolerant) teased our hair all up, leather jackets, bustiers/T-shirts etc; a friend of ours was a big cycler so drummed up some old bike handlebars and then... we tied portable music player to one of the horses. We timed it just so that we would be the LAST costumes in the class so when they made the last call we cranked up the volume on the player - naturally it was Born to Be Wild and we charged into the ring. It was awesome and so much fun - and inexpensive to do
Our barn always has a Halloween parade and it's so much fun. Last year, we borrowed a black pony for my son to ride in the parade. Our son dressed as Darth Vader (complete with a Vader helmet over his riding helmet and lightsaber). I made a black felt blanket for the pony that had the roll-down from Star Wars in white felt letters going up the back of the pony. It said "A long time ago in a galaxy far, far away, there was a boy who watched way too much Star Wars....)
This year our daughter, who just turned 3, is dressing as Dorothy and holding a Toto dog in a basket. She's riding my big chestnut mare, who I'm dressing up as the Cowardly Lion, and our son (7) is walking along side dressed as the Scarecrow. We always have a great time with the Halloween parade.
Since you guys want to do something together, maybe do a "famous couples" kind of theme, lol. I was intending to do Cleopatra last year and then wrap my horse in gauze and vetwrap as a mummy, so maybe you could do that and the other person be the Cesear with the roman horse like mine...or you could do Ken and Barbie or just do different themed Barbies and Barbie horses.
You could do Nascar outfits, paint big numbers on your horses and put "sponsor" stickers all over them and get some toy steering wheels to attach to the saddle, then go around the arena "racing" each other (slowly of course!). You could do Ichabod Crane and the Headless Horseman (my friend did that last year, turned out awesome!).
The best one at our barn for 2 horses was a bride and groom. Bay horse with rider wearing a black suit, and grey horse with the rider wearing a wedding dress with a loooooong train that covered the entire hindquarters and went to the floor. Horses had gold bands on their pasterns too. Looked spectacular.
Last year we did the horse as a bumble bee, 1 person as Winnie the Poo and the other as a honey pot. That turned out really good too.
Also have done a Heaven and Hell - one angel, one devil, and the horse was angel for the front half and devil for the back half.
Pegasus is always a good one. We researched and found out that Medusa is Pegasus' mother, so one of the walkers was Medusa complete with rubber snakes on her head. The rider was Bellarophon (sp?) - the only person ever to ride Pegasus (you can Google how it looks).
Have also seen 2 horses as hockey players. Coolers and polos in the team colours and riders in matching hockey uniforms with helmets, etc.
Seriously, a friend did the Bunny thing with her pony one year. They tied a pair of huge plastic boobs on the pony's chest (with a "bustier", he wasn't "topless") and put a pair of bunny ears on his head and a puffy tail on his bum.
For Hef, you'd just need a bathrobe, some slippers, and a fake mustache.