I wouldn't mind a new overland boss battle that gets you the very best mount
In all seriousness though, where I'm from, Oregon, we have a subspecies of elk called Roosevelt's Wapiti (Cervus canadensis roosevelti) that can definitely grow big enough to carry a man. The unrealistic part would be capturing one, at least if the AI had a better sense of how to pathfind the terrain. The game would have to maybe add making blowguns from bamboo stalks and poison darts from bad mushrooms so you could knock one out, and then maybe a way to rope prone creatures and drag them somewhere.