After a little bit of research, the key word here seems to be domestication.
If we're talking about it from a real standpoint, no animal is truly suited for riding. Some can carry much more weight than others but none of them has evolved with the specific purpose of being ridden by something, not even bred horses.
The first thing you need is for it to be able to carry a fair amount of weight, so that a rider doesn't overexert it. Camels and other ungulates might be one of the best examples due to their examples on good weight bearing capacity and the fact we've been riding some of them for a few centuries.
But the most important thing is doing to them what was done to horses. Horses and zebras are extremely similar species with a number of genetic similarities so great we can still breed them and have viable (although infertile) offspring, but the difference is that the domesticated horses are much more adapted to humans than wild zebras. Same for elephants. The problem isn't only that they're forced to carry unhealthy amounts of weight and work to exhaustion. They're wild, and many are literally broken from a psychological perspective to allow humans to ride them (lots of ethical issues towards shitty people working these to death by exhaustion exist, and for a reason).
So the key here is to:
1-ensure they are strong enough to carry weights superior to what you're forcing them to carry (I'd say make them ungulate and approach their anatomy to a Musk ox, since those can carry around 900kg).
2-domesticate them over hundreds of years to be used to human contact and interact with them more easily, as to reduce the amount of stress caused to the animal when humans ride them.
3-keep them healthy and attent to their needs. Not only it'll make it more ethical, it'll ensure better chances of happier, more relaxed and healthier animals.