This is the part that's 45%, so pay attention, it's fairly simple.
The odds that you'll have done it 77 times without seeing a drop are (according to that site, apparently, i don't have the patience to do the math at this point) about 55%. So, in a sample of 1000 people, around 550 will have not seen it after 77 attempts. Each time that you do it, the likelihood that you will see it becomes higher not per attempt but in the entire sample of your attempts.
In other words, it starts being really unlikely that you haven't seen it after a few hundred tries. That doesn't affect drop rates in any way, but it means that your probability of having seen it in x attempts, including this attempt will be higher. This is why trying a bunch of times will get it for most people.
One of the things about WoW that's unfortunate is, in 7 million players, there's like a .00001% chance that even doing it 100,000 times will never see it. Which, while it seems meaningless, is 70 people. So, probability states that about 70 people will never get it even if they do it once a day for the next 273 years.
Functionally, this means that even with very high drop rates, anything that has a random drop chance is, statistically, impossible for all players to get. In fact, a fair number of people (approximately equal to most people's total 'friends they've ever made in a lifetime' amount) will almost certainly have no chance to ever get it.
RNG adds excitement to the game, but always understand that it's existence means you may be one of the 70 people (from my example, it's actually a higher or possibly lower number depending on droprates) who will, after 273 years, never see it.
Therefore, be happy if you see it, because you could have been one of those 70 At least that's how I try to look at it.
- - - Updated - - -
Wow, just saw a lot of people with seemingly little math education arguing pointlessly.
Let me try to explain something here.
Your chance for this mount to drop is (according to existing data) about .78%.
No matter how many times you kill it, your chance to get it on that kill will still be .78%.
Your likelihood of getting it from any given kill will never increase (except eventually Blizzard will probably raise it to 2-3%, most things eventually have) above .78%
If 100 players kill the boss one time, the statistical likelihood that one of them will have gotten it nears 100%.
Now: what does this mean, and what does it not mean.
IT DOES NOT MEAN:
That you are more likely to get it.
That you are "due" a drop.
That it has to drop eventually or otherwise it won't actually be .78%.
IT DOES MEAN:
That your chance to be a member of the group "In the extremely small group of people for whom it didn't drop at least once out of 100 chances" is very small.
Now, to further this point.
You _still_ aren't any more likely to get the mount than you were before, because like someone said, you can't factor past events into probabilities for the future. Why? Because the chance of me waking up on November 18, 2013 is 100%. It already happened. That quantum state in this matterverse has already collapsed into a stable wavefront and is no longer accessible from my position in relative spacetime. It no longer has a probability. The cat is now either alive or dead. It is no longer a probabilistic event; "LurkerOnly wakes up on November 18, 2013" is, from my perspective, a collapsed event that is no longer accessible and can only be observed. It is 'actual'. I'm sorry, I know that this will completely cause some of you to think "this guy is insane", but this is just general relativity, 100 years old theories.
So, like I said. Remember that, in terms of probability, there is nearly 100% chance that out of 7 million subscribers trying once a day for 30 years, one person will never see it drop. That person could be you. Just be happy if you see it, and don't let your disappointment increase each time it doesn't drop.
In a very real sense, the next time you kill the boss and try to get the mount, will be the first time you've ever done it. Try to approach it with the same hope but understanding of its unlikelihood, and don't get too bummed out if you don't see it. .78% is kind of a low chance, after all.