I have read this a dozen times! I'm convinced that this is the correct answer. I'm suprised that so many people actually knew the answer! But I would like to add this....it is a really good idea to teach your horse to accept mounting from BOTH sides. You never know when you are going to be in a situation when you may need to mount from the right(especially if you trail ride). I happen to have a bad knee and HAVE to mount from the right side. I've never had one of my horses have a problem with it, but have heard of others that did. (Something with that whole left brain-right brain thing). I've also read that it has something to do with which side the mane hangs on. It is easier to pull on the mane from the OPPOSITE side it hangs on. Man, I LOVE trivia!!!!! :-)
The calvery wore their swords on the left side,thus makes sense to mount and dismount from left side.I train my horses from both sides,unless you ride with a sword it does not matter which side you mount.
I imagine the indians got on whichever side was more convenient! LOL! You also have to take into consideration etiquette. I don't know if this was actually a "rule" before people started riding horses for reasons other than work and war and transportation.
I mount mine from whichever side the ground is highest and easiest to get on. We short people have be more flexible to get up on a tall horse. LOL. Plus, who made the rule of which side is the correct side? lol
I think from about the beginning of man riding horse the primary use has been war. And up until relatively recent times swords and bow and arrow was the way it was done. And since right handedness is predominate. It makes sense that was how it began.