Like the one of a kind unicorn in TES: Oblivion?
Not quite like that; halla are the unicorns of Thedas. But maybe that's how you'd get a halla mount instead of a horse, or a bronto, etc. Which you could of course pass over if you didn't feel it was appropriate for your character. Or if we have starting racial mounts, it'd just be a more epic/awesome version of those. Or maybe it might be something else equally interesting, things that no one normally uses as mounts in Thedas. Wyvern, drake, etc--dragons being sociable creatures, it's not really that unreasonable that someone could tame them--maybe not terribly wise, but also not unreasonable. Domestication, though, is sort of out of the question, since only the high dragons seem to reproduce and it takes hundreds of years for them to mature into high dragons.
It's just a thought though.