A couple of years back, Mr. S (http://www.mr-s-leather.com/) had some unusual, elongated riot gas masks that looked extremely canine. By the time I'd found them, they were already out of stock. The item is completely gone from their catalog now, too.

However, the point is that I think they'd look very much like the masks you'd expect to see on an anthro with a long muzzle. The filters appeared to be on the sides as well, and were "embedded," for lack of a better word.