So after asking the four other people on my server with this, I am going to go ahead and assume until otherwise noted that whenever you kill Slabhide in Stonecore, you will get the drake. The only thing is, that you must Slabhide must be in the air to get this, and it is very easy.
The mount is Reins of the Vitreous Stone Drake - Item - World of Warcraft.
Boss will drop stakes from the ground, these do LOS, your heals and you, as well as dps and the boss.
Fire puddles spawn, and hurt a lot (they will be dust clouds before molten). Avoid these.
He will spit in a frontal cone, facing the tanking, and will hit every person not LOS'd, this will kill you or drain your healers mana almost instantly, LOS it, and the boss will not move.
When he goes in the air, you must group up, or someone is going to die, just burn the boss to 10%, and then only allow your tank to get the boss, wait for the next air phase, and burn it down popping cool downs then, when he drops, so does your mount.
Edit: Testing tonight on Heroic, we'll drop it in air phase.