I had an idea for a short story along these lines... basically, that the Europeans arrive in NA and find the natives are a horse nomad culture who really cause them problems when they try to take over land, far beyond what they did in OTL... the story would revolve around a person who comes to realize that (due to a passage in the Greenlanders Saga), that the Norse were the ones who brought horses to the natives...
Given enough time, imagine the fun if America could develop a horse culture similar to the Mongol hordes.