Go to the dollar tree they have lots of Christmas stuff cheap. You can get some inexpensive felt green or red felt to put over your saddle pad, decorate your saddle with garland use zip ties to hold it on. Put bows in the manes and tails. Get some reindeer antlers and attach the to your bridle, green and red polo wraps. Battery operated lights. I also take used a small dog collar sewed bells to it and put the bells around the horses ankles, use an old belt and sew bells to it to put on around their chest. Last year my girls were in a Christmas parade I took two santa hats cut a triangle of one to make it big enough to fit over their riding helmets then used the second one to fill in the triangle.
Have fun decorating.