My rarest mount is this one:
Which anyone can get easily just by playing the game, but only 7% do have.
The < 1% drop mounts I’ve had finally drop for me recently are much more common (Drake of the North Wind (16%), Vitreous Stone Drake (20%) and Pureblood Fire Hawk (24%)).
One thing to note: in the blurb on the site it says that not all player data is captured so those figures will be over-stating how common these achievs, mounts etc are.