the reasons why some animals are undomesticable have to do with their social structure (more or less). I remember an article in Discover magazine years ago that discussed this very topic, but I don't recall all the details as to why. Buffalo and zebra were specifically mentioned in the article, IIRC, as two species closely related to our domestic animals, but not domesticable. Onagers can crossbreed with horses, but they aren't really domesticable (although a lot of people in ancient times tried to do so). From what I remember, domesticability has a lot to do with a species' subservience to an 'alpha male', although that isn't the whole story. Horses and dogs were probably pretty easy to domesticate, as their social structure is very submissive to alpha males. Cattle and sheep were probably less so, but it was still done. Pigs and cats are somewhat of an exception, as they don't have herd structures. Cats became domesticated because it was to their advantage; they essentially gave up the usual wild skittish adult behavior and kept their kittenish behavior throughout their lives, in return for the vastly increased food availability around humans. Pigs... I don't recall just what quirk it was that made them domesticable; something to do with their intelligence. In any event, the article noted that nearly every species that could be domesticated has been, with a few exceptions.
That said, the POD here isn't all that huge... change the herd structure of buffalo so they can be domesticated. Would the NA be really so far behind? It's thought they came to the Americas 20,000 years ago or more... were the Europeans ahead of them in terms of agriculture and livestock?