From the link that you posted:
This novel armoured wolf is the reward for the achievements Ahead of the Curve: Garrosh Hellscream (10 Player) and Ahead of the Curve: Garrosh Hellscream (25 Player).
To complete this, you must have defeated Garrosh Hellscream in Normal mode prior to the release of the next tier of content."
Hell, even looking up the mount on Wowhead would have given you the same answer.
It says it RIGHT in the link you posted.
Yeah, as they said this is a reward for Normal Garrosh. It might be worth it to mention you need to kill him before the next expac to get it though and that everyone present during the kill gets one, there is no need to roll.
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No and frankly I find it a little stupid Alliance have access to it as well. Why do Alliance have access to Horde RBG mount but we can't get that awesome horse?
It would be dark inside my head...if not for the fires...