Most animals will bite you if they feel threatened or use as a last resort to protect themselves. Unfortunately there are also animals that use the threat of biting to control other animals and humans, horses are one of them. Horses will bite other horses to show who is in charge in a herd situation. They will also think that their human handler should be treated the same way as another horse unless they have been properly trained. While a horse bite on a horse is not that noticeable a horse bite on a human is very nasty. Here is a very graphic photo of a horse bite on a human.
I had a good friend once who was boarding her horse at the same place I was. Her horse was laid up for an injury so she leased this huge gelding named "Boogie." While getting him out of his pen one day, Boogie bit her in the neck. She has scars of the incident. Needless to say that Boogie was not leased by her any longer. It was the only time that Boogie did anything like that and he ended up being sold a few years later to someone who just loved him to death. Yes, horses can and do bite, but so do cats and dogs. You just have to be aware of it and know those training techniques.