IIRC, when they introduced Dungeon Finder, there was a real problem with people getting into Occulus and then immediately dropping because it was such a difficult instance to complete if everyone didn't know what the hell they were doing.
So Blizz added a bonus satchel of loots to the chest after the last boss, with the possibility of the mount in the satchel. AFAIK it was only on that Dungeon, and they don't do it any more.
Let me break it down further (without referencing patch notes explicitly, this is from memory):
Dungeon Finder Introduced in 3.3
Oculus was always a pain in the ass, but particularly so if people didn't know what they were doing, that it got to the point where people would just drop group if they got it as a random and suck up the time penalty.
In one of the patches after this became a well-known and widespread problem, 3.3.X, Blizzard introduced a special satchel that had a 100% chance of dropping if you completed Oculus in a random dungeon finder group. This satchel contained a pretty damn good chance (in mount drop terms) of containing the Blue Drake which was otherwise only attainable via rare drop chance from 10-man Malygos (which hardly anyone ran, and certainly not at that point), as well as a couple of badges and a few gems. So this was pretty much your only chance to get a Blue Drake.
This incentive actually worked out pretty well because MOUNT LUST!
So the system employed for the Oculus was adapted into the current Satchel of Exotic Mysteries which rewards players for filling a role that is currently in need in the Dungeon Finder system. Most of the time it will be for tanks (which is the most unforgiving role with randoms), though it does pop up for healers relatively often, and rarely, rarely is it for DPS. To acquire these bags with any frequency, you'll need to be a tank or healer (or both!)
I do believe, however, that there was a time that the Call to Arms system was implemented but before the Satchels with the mounts/pets were in. Then you would just get about double the money, and extra emblems. The satchels may have been a response to the initial difficulty of the Cataclysm heroics (I only ever queued with randoms a few times early on) rather than implemented from the start of Cata, though I'm not absolutely sure on that. You'd have to read patch notes from 3.3 to 4.1 or so (as well as relevant blue posts) to get the whole story.
Now, it appears as though you can still get the Satchel from Oculus, but you have to randomly queue for it, which means you have to be that level range. So you either get lucky, or you cap your level. With account-wide mounts in Mists, it may be a wise idea to get an alt at 80 (or up to 83 possibly?), lock their level, gear them up, and run random Wrath heroics, hoping that you hit on Oculus (and then hoping that you get the drake out of the bag). It's a long and convoluted process, but it'd still be quicker, if not easier, than running EoE every week. ALthough EoE now drops the Azure Drake regardless of the difficulty setting, though it used to only drop in 25-man. Not sure if the Blue Drake actually still drops from the raid or not.
Didn't even have the balls to reply to me even though I said it was my bad lol... on a better note while doing LFR Fall of Deathwing.. had 2 diff people open trade and give me firstly chest and than some random cloth gloves without any prompting.
