Ahead of the Curve: Garrosh Hellscream (10 player)

These "Ahead of the Curve" achievements are Blizzards solution to raiders feeling like their accomplishments were cheapened when a few patches later, players could go in and easily defeat bosses that once caused hours of frustration. The only solace raiders previously has was "Check my achievement date, I got that so far before everyone," while now these provide a small reward for those who managed to clear content when it was current. In addition with 5.4 Blizzard has decided to add a distinction between 10 and 25 man kills.

So far we have six achievements, including this one:
Ahead of the Curve: Will of the Emperor
Ahead of the Curve: Grand Empress Shek'zeer
Ahead of the Curve: Sha of Fear
Ahead of the Curve: Lei Shen

And in 5.4 Blizzard will continue and expand on this pattern with both Ahead of the Curve: Garrosh Hellscream (25 player) and Ahead of the Curve: Garrosh Hellscream (10 player). By adding separate achievements for 10 and 25, Blizzard hopes to distinguish between the difficulties in both. In addition to adding separate achievements, Blizzard also tied a mount to earning the achievement. Please note that the ten and 25 versions of this mount are currently the same.

These achievements were added on top of the "Cutting Edge" achievements introduced at the start of the expansion that were for heroic mode kills.
Cutting Edge: Will of the Emperor
Cutting Edge: Grand Empress Shek'zeer
Cutting Edge: Sha of Fear
Cutting Edge: Lei Shen
Cutting Edge: Ra-den

And, as said above, 5.4 Blizzard will continue and expand on this pattern with both Cutting Edge: Garrosh Hellscream (25 player) and Cutting Edge: Garrosh Hellscream (10 player).

These achievements are phased out over time, becoming unattainable when the new raid tier is released. Meaning that after 5.4 the FoS for Lei Shen and Ra-Den will no longer be available.

Note that the titles are not linked to these achievements so if you didn't manage to get Delver of the Vaults, The Fearless, or Storm's End you can go back and get them later!

This will no longer be available when the 6.0 pre-patch for WoD hits (source):

• The Garrosh Hellscream “Ahead of the Curve” and “Cutting Edge” Feats of Strength will cease to be obtainable.
  
  • This means that the Reins of the Kor'kron War Wolf awarded by the “Ahead of the Curve” Feat of Strength, will also cease to be obtainable upon release of the pre-expansion Patch 6.0.

I can confirm this feat of strength is still obtainable AFTER patch 5.4.7, got it on my toon on 24/02/2014 so a good while after the patch was implemented. My recommendation however would be NOT to sit around and put off getting it, WoD will be here before you know it and with it will go all hopes of getting that sweet mount ;D Happy Garrosh thwacking!