Most Horses have a naturally inbuilt fear of horses, they'll often react just to the smell of them. However its possible to get the horses accustomed to pigs and to desensitise them. I have a lot of horses (i breed them) and have just got my first pigs, i think its great whilst the horses are young, to get them over their fear! I've put the pigs basically between the horses fields and the stable block so they have to walk past them to come in, and also as the horses are currently getting fed at their field gates, the pigs are probably only 10meters away from them. So far, no reactions that weren't expected, just a couple of the ponies are standing sniffing the air! i'm not anticipating too many issues. I think the best thing is to make it so as you lead your horses to their stables, they can definately SEE the pigs, so they don't get too startled.