A caparison is by definition a decorative covering for the tack or harness of a horse. It can be made of leather or fabric and embellished with metal, bells, tassels, dags, emblazons, mottoes, etc. You may wish to consult "The Hunting Book" by Gaston Phoebus, and "A History of Horsemanship" by Charles Chenevix-Trench(p. 99, & pp. 106-107) for pictures of caparisons.
You will need:
If you use heavy weight fabric and/or line and top stich your caparison, it will greatly add to its life expectancy. Definately plan on your caparison getting muddy, dirty and torn if it is cloth. Machine washable, durable fabric is a must. Also plot out your design on paper before cutting your material. Be sure to add 5/8 inch to your measurements for the seams.
The easiest place to start with your caparison is to make a blanket that will cover your normal saddle blanket and go under the saddle. I recommend making the blanket at least 22 - 25 inches wide. Since the horse I based my measurements on is a 16 and a half hand tall Appaloosa mare, some of these measurements may need to be scaled down on a smaller horse, and scaled up for a larger horse. The total length of the blanket is 100 inches. You may wish to shorten this to 80 inches or less if you have a smaller horse, or if you prefer a shorter blanket. Wait until you have a saddle or borrow one, before you cut the slashes for the girth, since the blanket won't lay right if the slashes are too far down and the girth will pull up if the slashes are too high. The girth slashes will need to be about 5 to 6 inches long. If you don't have a saddle handy, you could set the slashes anywhere from 15 to 18 inches down on each side. However, it's better to wait until you have a saddle to get the best results.
Measure around your horse's chest from the front edge of the blanket to the other side, allowing for drape. Be sure that the chest band will be loose enough that your horse can move without ripping it off, yet not so loose that it ends up at his knees or interfers with his leg action. On the front edge of the saddle blanket, I started about 14 inches down and marked the blanket there, then made another mark 14 inches down on the other side. The measurement for the chest was 54 inches. It helps to use safety pins to anchor the cloth while you experiment with the angles, I highly recommend that you bring along a friend to help.
For the hindquarters, measure down the same distance on the back edge of the blanket as you did in the front for the chest band (that would be 14" for this horse). Then take your tape and measure in a straight line to the horse's tail. Take off about 3 inches from that measurement, since you don't want the caparison fabric directly under the horse's tail. My equine model has a big rear end, so my measurement was 38 inches from blanket to tail. The strip of cloth that goes over the hindquarters and hangs down, can be the same length as your blanket if desired. You will need to visually center where you want the hindquarter strips of your caparison to cross. I set my cross section 21 inches down from the center of the hindquarters, and 16 inches over from the back edge of the blanket and leaving 22 inches to go to the tail. You also may want to have a strip of cloth connecting the top of the blanket to the top of the cross section, so you caparison does not go sliding off your horse's rear as he moves.
Use heavy cord to connect the caparison together. Either set eyelets or use the button hole stich to finish the holes for the cord to go through.
Reincovers need to be about a yard long. It is better to make one for each side, rather than one long strip, so you can control the reins directly instead of having them slipping underneath the cloth. Paper binding clips can work to keep the rein covers in place.
Caparisons are a wonderful way to deguise mundane tack and add heraldic flair to equestrian events. They also can dramaticly dress up a less than attractive horse. Caparisons can be as simple or elaborate as you wish. There are many places to look for ideas: art books, statues, paintings, manuscripts, etc. Note the two books mentioned at the beginning of the article for starters. Good luck, happy sewing, and safe riding.