Head masks for horses are common throughout history. Celtic, Anglo Saxon, Etruscan, Assyrian, Greek, Roman, and Chinese are all examples here. It helps to remember that history is usually the preservation of the hard stuff. Often when you see examples of horse tack in museums, it's either a recreation, or the real thing without it's cloth or leather. Stuff happens.
Like when you drop a "t" into the word Saxon that doesn't belong there... har. Me type pretty one day.
Other references (and typos) for drawing historic horses can be found at htdhorses.com