This is a massively confusing and mis-diagnosed or just non-diagnosed issue in horses. There are actually several proposed causes for this, which I thought we would go through.
Equine self-mutilation is typically characterized by sudden, violent outbursts of the horse towards himself. This can include kicking, biting of the shoulders, chest, or flank, or some combination of these. It can happen in conjunction with squealing or even throwing themselves to the ground as well.
The first potential cause is self-directed intermale stallion aggression. When a stallion is kept from estrous mares, he often becomes aggressive. In the wild, this aggression would be directed towards the males keeping him from the mares, but as we generally keep stallions separate from each other, they turn on themselves. Once it has started it can be a very difficult cycle to break.
The second potential reason is pain, especially with biting at the flanks. This pain is typically digestive, and the horse is biting to attempt to attack the perceived cause of the pain.
The third reason is an unnamed Tourette’s-like syndrome in horses. This causes repeated, stereotypic, useless behaviors like self-mutilation. This has not been studied in depth, but seems to present independent of stress, and at a higher frequency while stressed.
No matter what the cause is, horses are not meant to bite themselves. If your horse is chewing on himself beyond just itching, there is SOMETHING WRONG. It is not cute or funny. He is distressed or has a neurological issue. Please consult your vet to attempt to treat it. Did you know this was a thing? Comment below