Did you do any gradual introductions or just put them together? Being herd animals horses like to pick out their pecking order, and if your new guy is acting too big for his britches your other horses may feel threatened. Are they able to see each other from their separate pastures, and touch noses over the fence? If so, allow them to do that for some time, and take the less aggressive of the two and put them with the new guy, it's not unusual for them to test each other, maybe do some 'on halter' intros. Good luck!