The Chanfron, Chamfrein, or Shaffron took it's denomination from that part of the horse's head it covered, and was a kind of mask of iron, copper, or brass, and sometimes of jacked leather, enclosing the face and ears, some of these chanfrons seem to have been contrived as to hinder a horse from seeing right before him, perhaps to prevent his being intimidated by any object against which he might be directed, so as to cause him to start aside, or lessen the celerity of his charge.
Source: wiktionary