Horse herds that have been established for a long time do not readily admit new horses. Let the new horse have fence line contact with the established herd for several days to several weeks (depending on the severity of the problem) before turning the new horse in with the established herd. Make sure your fencing separating the horses is strong and safe because the horses may sniff noses, squeal, strike and kick at each other over the fence. When the established herd ignores the newcomer and tolerates its presence along the fence, it is probably safe to turn the new horse in with the herd. Make sure that your pasture is big enough for the new horse to escape from the established herd, if necessary, and watch the horses closely for a day to make sure they are accepting the new horse.