Rarity. Mandatory addon for all mount/pet/toy farmers.
This sound so painful. Then again, it's not that unrealistic for it to happen to someone I guess. Whenever I see these kinds of attempt counts I can't help to think that maybe a portion of those attempts were unelligible for mount loot for some reason, but that's just not verifiable.
An extreme example, but I am still trying for Sha and Nalak every week, and I'm nearing 7200 attempts on both. The supposed drop rate being 1 in 2000, so that's 3 times the drop rate +already. It's safe to say I'm very unlucky on those. Can't give up after that many tries though...they will drop eventually.