I expect that the lesson pony described by OP is “lazy” because there is some back or hock soreness, and pushing a horse like that into a canter by force is going to provoke a bucking response. We tend to think that ponies are naughty when they act out, while the high dollar show horse must have something physically wrong that needs diagnosis and treatment. If this general scenario occurred with a TB or WB we would be saying get a vet workup.
We make a similar difference between children and adults. If a child is anxious or complains of discomfort or imbalance, they are being sullen and insubordinate and need to suck it up and do what they are told. If an adult reports the same things, they need to see a doctor, a chiro, and a sports psychologist to solve everything.
I hated all sports instruction as a child and have enjoyed it generally as an adult. When I started doing dance based fitness, aerobics, zoomba etc starting in my early 20s I loved the “go at your own speed, listen to your body, don’t get hurt” advice. Absolutely did not exist in high school PE.