Java Edition:Equipped Wild Horse
Normally a player must first tame a horse before they can equip it. However, because of a bug,[1] you can put a saddle or iron/gold/diamond horse armor on a horse that isn't tamed. There is a total of 245 distinct horses that one can obtain.[2] You should not ride the horse with the armor or saddle in general because this could result in it becoming tamed. But riding an untamed horse with a saddle will crash the game in some versions[test] if the horse tries to become tamed.
Obtaining
In 13w21a to 13w21b, you must find a wild horse and then ride it, shift-click the saddle/horse armor on and get off the horse hopefully before it is tamed. If you are kicked off the horse in the process of equipping the horse armor or saddle you will still be in the inventory and can add the items.