Over the millions of years of evolution, the horse has evolved as a prey animal and its first response to anything scary is to run away from the threat. This is known as a flight response. Horses understand that their safety relies on their ability to put distance between themselves and something they perceive as dangerous.
If a horse is prevented from being able to run from the danger, his second instinct would be to fight. He then may feel forced to bite, kick, rear or exhibit some other unwanted behaviour. Another reason a horse may “fight” is if he is in pain or is forced to do something that is painful. For example: Bucking could be the result of a poorly fitted saddle.
So what is dangerous to a horse? Well if you ask them, they’ll say almost everything! Their well-being relies on their perception that everything that moves or is new to them could eat them. So when we lead or ride a horse up to or past something it has not seen before, the horse can become wary or nervous.