It truly depends on the horse. If you have a social horse they will most likely sniff the other horse, it could get a little exicted so be careful. If the horse dosent like other horses it might...
well most horsers are very cleaver and if you treat your horse right with love and care and have had it a long time and spend a lot of time with it then you
You can get to know your horse by hanging out with him - by grooming, scratching and rubbing him. Look at horses in paddocks - how are they friends? They 'groom' each other with their teeth!