When you get a new horse, you will inevitably put it in with the rest of the horses that you own. Much like with other species the pecking order will be altered with a new arrival. Here are some ideas from Joanne Meszoly of Equisearch.com to make it so it won’t be as awkward for your herd or your new horse:
Let the new horse run around first…separately
Let the new horse get familiar with where he will be most of the time before you let the others go in the pasture. After you see he has acclimated himself, let the others go in one at a time. If the new horse is familiar with the territory, he will be at less of a disadvantage. It will also be better if you do this during the day so the horse can also see the lay of the land.
After mealtime
Horses are going to be a lot less anxious after mealtime, so that may be the best time to introduce your new horse to the crowd.
Two isn’t a crowd
If you have two new horses, let them become used to each other before you let them into the new herd.
Source:Equisearch.com