Why Do Horses Fight?
Horses are usually more inclined to flee from danger or threat rather than fight. However, when flight is not an option, then a stressed or angry horse will fight to protect itself. This is a very strong instinct and it is still very present in domestic horses.
In addition to the flight or fight instinct, horses are often driven by their herding instincts. As naturally herding animals, most horses are sociable and will quickly fit into some sort of pecking order amongst the other horses in the surrounding area. Some are just naturally more dominant, while others show more passive behavior and are happy to be the followers.
In most cases, horses are not fazed by their level on the pecking order and will happily accept their position without much fight. However, there are the odd occasions where a horse will protect its territory or assert dominance on another and this can often result in a fight.
While serious fights that result in injury are not very common among horses, the occasional squabble can be expected, especially if one horse is aggressive or temperamental by nature. Horses may also feel the need to re-assert their dominant level in the pecking order with the occasional lunge, or pinning of ears around another horse.