By any chance is this pony getting fed concentrates? Does the hay have a lot of protein in it? Are people sneaking in sugar cubes to try and get the pony to be friendly?
All the above can be rocket fuel to a pony.
I am all for getting an advanced rider on the pony, child, small teenager or a small adult. Do you have any ex-jockeys available?
I owned a POA who was 12.2 hands and ruined. This little helion tried to scrape me off on a barn door the day I tried him. All it took was applying the correct aids at the correct time and to the pony’s surprise he was obeying me (and believe me he was surprised, I was supposed to be on the ground writhing in pain according to him). I just gave him a few rides to remind him that he needed to listen to the aids.
He was never a perfect pony, nowhere close. If I could have stood riding his extremely short strided rapid trot I could have gotten him further but my legs are too long to ride the trot he gave me (I think my thighs were too long for his build.)
I did succeed on getting a 13.3 hand British Riding Pony to become a decent ride, but he had more of a horse’s stride instead of a pony stride. This second pony had extremely well-sprung ribs and he fit my legs well.
Ponies resent being hit, they resent it deeply. It is necessary for the rider to find another way to get the point across.