A herd of wild horses will include animals of both sexes and all ages. In a domestic situation, geldings and mares can be mixed in a field, but this is more likely to lead to dominance and leadership disputes, so they tend to be segregated. Stallions are usually kept away from both mares and geldings because it is natural and common for stallions to collect and mate with mares and fight other male horses. Within any group there is a hierarchy and each horse is responsible for protecting those lower down the order. Horses prefer to be in a group with other horses, even if they aren’t familiar with them. The instinct is so strong that grazing horses will sometimes make attempts to escape and join horses that pass by.
Most fights between horses are usually triggered by a need to preserve a certain position in the hierarchy, or a desire to challenge another horse and, in turn, gain a higher ranking within the herd. As a result, there is more ritual posturing than physical contact in the confrontation. A fight will take on a new intensity when a stallion has acquired a harem of mares and is challenged by an outsider. As a general rule, horses are not aggressive; they are more likely to flee a situation rather than engage in a fight. Even in a group of only mares or only geldings, there will be horses that establish dominance within subgroups. A dominant horse will try to bring more members of the herd into their group, so confrontations may occur between the group leaders.
The herd is an extended family, and many collectively contribute to the care and education of foals. Young horses naturally gather together and the nearest adult will usually take the responsibility of looking after them. This is more of a means of establishing dominance over the young, rather than a concern for their well-being.
Some horses within a group may develop strong attachments to each other. When they are stabled, if one horse is taken out and another is left behind, the one left behind may become very upset. If two good companions are turned out together, they will usually stay fairly close. If one of the friends dies or is sold, the horse may show signs of depression or grief.
In a herd, stallions round up their mates. This is to ward off the slightest sexual threat from a rival stallion, rather than to protect the mares from danger. In many domesticated groups, horses of either sex will round up horses that they dominate as part of their efforts to reinforce their dominance.