This is a habit with more impact on the human than the horse! Constant vigilance is necessary with a horse that bites. Any horse might bite, especially if you’ve already ignored other warnings. It is imperative to see a bite coming beforehand so you can get out of the way or block it. If you choose to try to block the bite, try to meet the horse’s mouth with an elbow or the length of your forearm so the horse has the mildly unpleasant experience of bumping into something hard with his lips or cheek and cannot do significant damage. You may have to block a bite if you are in a space that does not allow you to quickly get out of the way. Do not swing your arm to hit the horse, but rather to block them. Many horses will abandon the behaviour when they see you raise your arm, but hitting the horse will be seen as an affront (which it is!) and may even exacerbate the behaviour.
While the temptation may be strong to respond to biting with anger (including hitting), this reaction can only makes things worse. Generally, this type of reaction teaches the horse more about getting away quickly than not biting and often incites increasing aggresiion. However, a firm voice and at most, a slap on the neck or shoulder may discourage non-confirmed biters. This is a more standard response to biters. You may elect to tie a horse that bites to avoid the issue and protect yourself.
Horses that aggressively and regularly bite other horses or animals need to be pastured and stabled alone. Muzzles that allow the horse to eat and drink can be employed. The environment and routine that the horse is kept in should be examined for cause, however the habit is often established at a young age and can be difficult to correct.
Self-mutilation is rare in horses but does occur. It is generally considered a symptom of a neurological disorder but may be a response to terrific pain or agitation.