Przewalski horses fall under the same category. They cannot be ridden. Even as young animals, they freak out when something is on their back and will do anything to get it off, haha. Zebras are the same way. There is one case I know of where a zebra is considered 'almost' tame. This gentleman can ride it. It obeys commands from the reins and legs. BUT.... Every now and then it has a bad day and will not continue with it. The zebra is also said to require daily interaction, otherwise it will progressively get more and more 'attitude'.
However, they've realized that it only takes four generations for foxes to tame down enough to be household pets. So that's pretty cool!