The two most important strategies for preparing your horse to wear a costume are Approach & Retreat and Simulations.
Approach & Retreat
Rather than focus on getting the costume on the horse, turn the costume into a game. Break the costume up into as many pieces as possible and play with each piece individually. Put the pieces on a nearby bench or on the ground and ignore it while you play with your horse. When he’s warmed up, start a game of touch it — drive him around and have him put is nose on things — and include the costume as just one of the things to touch. You can put a cookie on it to make it even more interesting.
Once he is comfortable with the costume lying still, pick it up and walk away from your horse. Let him drift to the end of the line if he needs to. Walk around until he relaxes and starts to get curious. At that point, you can let him approach. Allow him to retreat when he needs to and repeat the process until he is calm.
Keep approaching and retreating as necessary until he can stay relaxed while you rub him with each piece of the costume. Then put each piece on and off many times until he feels comfortable.
The next step is to move your horse while he wears a piece of the costume. You can then add more and more pieces until he is wearing the entire costume.
Make sure that you can remove the entire costume quickly at any time. Leave buckles unfastened for quick removal.
If your horse is not relaxed during this process, you need to do more simulations.
You don’t have to start out with the actual costume, especially if it is fancy or delicate. Find ways to simulate the experience for the horse. Simulating can be the most powerful way to desensitize your horse.
If the costume has leg pieces, you can have your horse wear splint boots or bell boots, or use polo wraps. Simulate a cape by using a folded blanket or even a Navajo saddle blanket, gradually unfolding and making it bigger and draping it over more and more of the horse. A fly mask with covered ear pockets and a nose piece can stand in for a hat or mask.
Use tarps, plastic bags, car keys, sleigh bells, and other noisy items to expose your horse to the sounds he might encounter at a Halloween play day.
Remember to use approach and retreat when you introduce each new stimuli. And don’t forget to give your horse time to get used to your costume too!