They use to call it bridle lame many years ago, because the pony would only go well on one diagonal at trot or canter and stiff or lame on the other, took time to get them going well on both diagonals at trot or canter.
What a lovely animal and a lucky boy as his guardian angel must have been looking after him.
My old TB had 7 owners in 2 years before I bought him and I had him for 11 years before he was put down with bad arthretic hip