Reason behind the behaviour.
There is always a reason for a behaviour and every
behaviour has a reason.
Designed to survive, horse behaviour is as close to
their natural ancestors as it was at the beginning of their
time. Remove their tack and turn them loose in a herd and
they will behave as horses always did. Man's involvement
in specific development, for example speed in a
Thoroughbred has in no way infringed on a horses natural
behaviour and that number one behaviour is survival.
Social Dominance, scientifically known as 'Redirected Hierarchal Behaviour' is survival behaviour. The more dominant the horse, the greater it's chance of survival & reproduction in the the wild. Therefore it is in a horses own best interest to be the more dominant in the relationship, whether that be with a human or another horse. This can look as obvious as barging, double barrel kicking or the more subtle act of making his human move their feet ever so slightly.
Horses, as I am sure you know, are very capable of learning certain "command" words - whoa, stand, click click etc.... Frequently, horses are required to guess what is expected of them, often being chastised for guessing wrong, leading to a confused and unsettled horse.
Below is an everyday example of how a horse so often has to guess what we mean.
This horse is being asked to pick up his foot.
Owner - Squeezes fetlock and says 'pick up'.
Owners friend - Wraps fingers round pastern and clicks her tongue.
Owners daughter - taps foot and says 'up'.
Farrier - Squeezes chesnut and says 'foot'.
Yard groom - pokes leg with finger and says 'this one'.
All of the ways the question was asked were acceptable, yet that same horse has to learn that one question, asked in 5 different ways.
That's like us having to learn 5 different languages.
Pain & Fear
A horse can only communicate pain, aside from obvious lameness, through his behaviour. Whether that is flinching when we touch a sore spot, or rearing when asked to do something that hurts, as well as the endless amount of creative avoidance tactics that horses teach themselves.
Learning to avoid pain by adopting a certain behaviour doesn't necessarily mean the behaviour will return to normal once the pain has been treated. Once a behaviour has been learned, for whatever reason, it then has to be unlearned.
Something I see a lot of, is horses making decisions. This is a form of social dominance. Allowing horses to make their own decisions, however small and insignificant can cause problems later on. Such as when that horse starts to make decisions that aren't so small, for example, loading in a trailer, or being caught.