True about white-tailed deer vs. elk, moose and reindeer which run more like horses. The picture you posted was of a white-tailed deer, so I wanted to mention the difference in movement. I think an elk variety would be a great mount. However, how fast are they and what kind of stamina do they have? Moose rarely out-run wolves, wading into water for escape to my knowledge. Are elk better runners? I'd still make them apaloosa or something to differentiate their breed as something special. Also, elk are huge. It's be akin to riding a clydesdale into battle. I think you're on the right track, choosing something akin to what's on earth but changing it to fit your needs. I only mentioned wildabeests because they're something other than a buffalo, horse, ass, etc. They're related to buffalo but obviously are much different. You could invent a close cousin for a colder climate, maybe cross it with a musk ox or something. Depends how much reality you want in your fantasy or how much into the realm of fantasy you're willing to go.