Wow. I was surprised to see this thread resurrected a full year after I'd started it.
re: the idea that this is some kind of magic, I think that distance from the animal has to be taken into account. I know that distance is not taken into account in other communications (for example, no matter how far away I am from my men, I can issue a "follow me" command and they'll come to where I am) but I think it would certainly be realistic to expect a horse to come when called if (1) the horse can hear you, i.e. distance, (2) you've worked with the animal long enough to establish communication, i.e. time riding (3) the horse and the rider are skilled, i.e. the riding levels.