It started with mail carriers on horses, but not sure if Postal carriers were the first to name Posting on a Horse. Who develop and name it?
It appears there are different assumptions about its origin, the following extract suggests the more common ones:
1) The Postman - This story suggests that postmen carrying mail throughout different parts of the world would prefer using big, long-strided horses for speed. Unfortunately, these fast horses were also bouncy forcing the men to learn how to ride the trot by bouncing up with every other stride as opposed to bouncing with every bounce of the horse.
2) - The Cavalry Man-The second theory implies that posting was invented when the cavalry men posted the trot for endurance's sake. Posting up as the horse's right leg was stretched out front would work one side of the horse's hind legs more than the other. So, the rider would ride on the right diagonal (up as the horse's right front leg was forward) for a while and then switch to the left diagonal in order to optimize the amount of work each leg was expending.
3) - The Postilion-The last story says that the fanciest of European royalty and aristocracy would opt to have a horseback rider driving the carriage/coach's team while mounted on one of the pulling horses as opposed to a low-life coachmen riding up top with the rich folks. Eventually, these "post boys" discovered it was much easier to stand up in their stirrups with every other bounce of the horse as opposed to getting jostled around. "As time went by, 'posting' became the accepted way for aristocrats to ride their fancy horses up and down the urban bridle paths like Rotten Row in London or Central Park in New York City during the Victorian era."
The third theory, the postillion rider, appears as the more credible according to the Manor:
It helps to understand a little about history and how all this moving up and down at the trot got started. When European royalty and aristocracy rode around in their carriages a few centuries ago, there were all sorts of things they did to indicate who had more status and money and all the things people put so much store by. One of those things was where the coachman sat relative to the swell folks he was driving around. If you were really swell, you didn't have a coachman at all. Instead, you had postillion riders who rode on the near horse of a pair and guided them both from the saddle.
Now the swells tended to like fancy, animated horses that trotted down the road with their heads in the air and popping their knees and just looking pretty proud of themselves. All that proud, bouncy trotting could jar a rider quite a bit. So the post boys figured out that if they flared their legs out and stood straight up in their stirrups every other stride, they didn't get bounced around quite as much. They pushed off on the balls of their feet so and their heels came up every time they stood up on their toes to avoid the bounce.
As time went by, "posting" became the accepted way for aristocrats to ride their fancy horses up and down the urban bridle paths like Rotten Row in London or Central Park in New York City during the Victorian era. And eventually it became the riding style adopted by people showing horses with high knee action like American Saddlebreds or Morgan and Arab park horses.