This is a video of some wolves running. Focus your attention on the wolf's shoulders and back. Do they stay straight? No; one notices that they buck considerably. The wolf's haunches move upwards and forwards as it runs, while its back depresses. This would make for an extremely bouncy, unstable ride for any potential rider (which could only be the size of a human child at best - if you've ever seen a child ride on the back of a large dog, it's the same sort of thing). Now, take a look at
this video of a horse galloping. Notice how its back remains almost perfectly horizontal.