Horses walk the fence because they’ve been allowed out into a new paddock with which they are unfamiliar. Their first instinct is to set off on a territorial patrol which means walking all around the perimeter next to the fence to learn the scope of their home range. When they’ve completed this task, they might settle down to relatively quiet enjoyment of their allotted space.
Sometimes horses pace next to a fence. This is indicative of a lack of stimulation and an environment which is unsatisfactory. Pacing is a kind of displacement activity intended to calm and distract the individual from the stress of being confined to an area which is much too small for his instinctive needs and without stimulation.
Horses naturally have home ranges of 30-80 square miles. So even large paddocks are a tiny fraction of the space that they naturally desire. However, it is said that they succumb to this compressed space and accept it. They give up hope of being territorial.
If they live in a large space, they won’t defend intruders but if they live in a very small space, they may be tempted to defend intruders. This would seem to be because space becomes a prerequisite. It is more important to them and therefore they hang onto what they got by defending it.
You can see bands of wandering horses showing no defensive behaviour of their home range and they may even share grazing zones and watering areas without disputes between themselves and intruding horses. This may happen when two small herds meet in a large enough area when they simply avoid one another and get on with their lives. But with the pressure of crowding, the “territorial imperative”, as Dr. Desmond Morris calls it, kicks into action.
Although some horses switch off their territorial desires due to living in such a cramped environment, when domesticated, some individual horses may defend their space fiercely in a small paddock.
People working with horses in paddocks need to take care when a new horse is added if there is a resident horse who has been there for a long time. The resident horse may become territorial aggressive due to a release of pent-up territorial defensiveness which has never been expressed and lash out savagely at the incoming horse. Such a horse may have a long-standing desire for a larger territory and therefore cannot accept an intruder.