I used to teach and compete, many years ago! I haven't ridden for over 15 years. We used to have a marvellous pony for my ds1 (who is now aged 33, ds, not the pony who alas is long gone to the pasture in the sky).
You could put a tiny tot on that pony, and he would walk along as if carrying a cargo of fine china, but if I or the stable owner would get on board (as we would in the evening when taking the horses up to the top field, riding bareback, halters, not bridles, ride one, lead two) the little bugger would put his head down and take off like a miniature steam train, just because he could!
That pony was a real character, I swear he could have won a bending race without anyone on his back, just put him on the start line and let him go!