Just as every horse is unique, every herd of horses is also individual. Horses have very strong and individual personalities and some are naturally more aggressive than others and some horses are completely submissive. The personalities of these individuals will determine which one is top horse and which are the followers. This ranking order is necessary for the way a horse is psychologically wired, they need a leader at all times. When you work with your horse, you are viewed as top horse.

When you introduce a new horse, your "top horse" can show signs of aggression, as horses can be very territorial. If your new horse is also aggressive he may challenge that horse back for the rank of top horse in the herd. The addition of a new horse to the herd generally means that all the horses will engage in competition to see where they fall in the pecking order of the herd.

Watch for signs of aggression such as ears laid flat, a horse that holds his mouth open with his neck craned forward, eyes wide open is another sign of fear and/or aggression and if the tail is held high and swishing violently, watch the legs of your horse as he may pull one up to kick at the new member of the herd.

If at all possible, introduce your new horse slowly. This will keep the herd from being upset and it will give the other horses a chance to get familiar with their pending herd mate. Keep him corralled next to the herd and let them see and smell each other. When the other horses seem to be accepting your new horse, introduce him to the herd one horse at a time so you can see which horses are going to have a hard time adapting to a new herd member.

In most cases, corralling your new horse next to your old horses until you fully introduce him to the herd will reduce the stress of accepting a new member to the point many of the horses will admit their new herd member without much fanfare or fireworks.