A: Introducing two horses is something a lot of horse owners don’t relish, and adding a new horse to an established herd can really up anxiety. I don’t know if there’s any one best way, because not only do farm resources differ, but individual horse temperaments play a role, too.
This is not just an issue of domesticated horses. Feral horses have introductory events, too. We see posturing and sometimes fighting between harem and bachelor band stallions at meet-ups or challenges. Fillies usually leave their home band and join another, and agonistic encounters with established members can occur until they settle into the hierarchy.
Domestic horses living in groups form hierarchies that are linear but often with triangulations, and also the dominant horses might vary according to resources and activities. Once established, a hierarchy tends to remain stable and most agonistic encounters we see are threats or avoidance behaviors. Not only do we see dominant and subordinate relationships, but we see tolerance or preferences by a dominant with more subordinate individuals.
Even a pair of horses will establish some kind of subtle or overt hierarchical arrangement.
Introduction of new pasturemates can be done with some care to try to minimize risk of injury or displacement around limited resources, such as food, water, and shelter. The two can be introduced across adjacent stalls or a safe fence. But these introductions are just not the same as when they can physically and freely interact. I prefer to just put them out together right away. Once introduced, unless they are both low-key horses, you can expect to see threats, but also likely some chasing, biting, and kicking (aggressive or defensive).
Ideally, the horses will be turned out in a very safe environment: sturdy fence that minimizes the chance of a leg getting entangled, a spacious area so one can get away from another, supplemental feed (if used) dispersed over a good distance, space around the water source, and, if a shelter is available, one that is large enough to easily accommodate two horses. Many people recommend removing hind shoes, but a well-placed strike can argue for removing front shoes, too. Be sure both horses are sound and in good health, both so they can withstand some aggression, but also to avoid sharing contagious diseases (don’t forget that part!).
Realize that when horses are kept together only part of the day, to some extent there will be re-establishment of the relationship every time they initially go out. This will lessen over time. The safety practices I mentioned for the initial introduction should continue because the more aggressive interactions you see will occur most commonly around limited resources.
When introducing a new individual to a herd, the same recommendations apply. But especially with a group, really emphasize lots of space! Some have tried manipulating the group temporarily, such as removing the most aggressive individuals for the initial introductions, or allowing the new horse to buddy with just one or a few for a little while. These tactics might dampen or only just delay the ultimate severity of the aggressions.