I just saw a vendor that sells mounts, but you need an item called vicious saddle to buy one. I searched on wowhead and others how i could get one, but it’s too confusing as i’m a fairly new player.
You must reach 1400 arena/rated BG rating, then fill the progress bar with wins in the bracket(s) you’re 1400+ rating in to earn the seasonal mount. Then you must fill it again to get the saddle. As far as I know you can only do this during an active PvP season (though I might be wrong, please correct me if so), so time is running short if you want it before SL.
EDIT: You can’t earn progress towards the mounts in random BGs btw, even if you have the rating. It has to be in arenas or Rated BGs at 1400+ rating.