I'd always heard that it was because knights had their swords on the right side, and they'd hit the horse if they mounted from the right.
Or was it the left side......i can't remember.
It may just be a myth though. Posted via Mobile Device
I also heard the knights and sword thing. I heard it was because the sword was hung on the left side of the human so they could grab it with their right hand. Which means when mounting you did not have to toss this long, heavy sword over the horse.
It's because when horses were originally used in ancient warfare, it was too difficult for someone holding a shield/wearing a sword to mount from the right. So the left side became the 'proper' side to mount a horse.
Also the use of a left hand for writing or sword play was considered a "sign of the devil". Dexter is latin for right and Sinister is latin for left. Which is why the twin on the left side or the left handed twin in a pair of identical twins was considered the evil twin because it was sinister and thus working with the devil