Also, I forget where I found it, but there is a site that you take a base drawing of a horse and pick from a large range of colors and coat patterns to make it look just like your own horse.
You then can add mane/tail colors and style (english or western, long, braided, etc..) body markings and whatnot.
Once you had the horse done, then you could pick whatever style and color of tack, saddle pads, boots, stable blankets, and accessories and try them on "your" horse.
You could try a google search for the site.
Ps. Love the t-shirt idea!