I’ve always imagined the first person to ride a horse as some surly 12 year old who was sent out in the cold and snow to milk the mare. “Screw it,” he said, as he turned over the wooden pail, and impulsively jumped on the horse’s back. Startled, the horse took off, with the kid barely holding on by the mane. Then, “Wooot!”
That is probably about how it happened. There is a similar story about Alabama hero John Pelham and a bull he passed by on his way to his one-room school. He was a very rough boy before his West Point days.