Hearthstoned

How do you get this feat of strength? As of 3/11/2014, we now have our official answer from Hearthstone Patch Notes 1.0.0.4944:

The Hearthsteed mount reward has been added and can now be obtained for your World of Warcraft account! Win 3 games in Play or Arena mode to acquire one of your own!

The mystery is finally solved! It was originally believed this Feat of Strength would be rewarded for purchasing a card pack in the Hearthstone beta store. However, Blizzard Community Manager Zeriyah later clarified that the "special gift" would actually be a golden version of Gelbin Mekkatorque, available throughout the closed and open beta tests.

So to get this FoS you must get the Hearthsteed mount and this will reward the Hearthsteed mount.

xD

Please note that as this is a promotion, the mount and achievement will likely only be obtainable for a short time, so if you want it, you should act before it is too late! Zeriyah has stated on Twitter that the achievement for the [item=98618] is NOT a limited time offer.

To celebrate the official release of Hearthstone: Heroes of Warcraft, all players can participate in a few launch day activities, as well as unlock the [item=98618] mount in World of Warcraft. The mount is rewarded for playing, and winning three games in play and/or arena mode. Games won before the public release patch went live does not count towards this total.

After you have won three games, you will be awarded with the [hsachievement=110] achievement in Hearthstone, and you will receive an ingame mail in World of Warcraft from Mei Francis with your [item=98618] mount attached.

<Name>,

This fine magical construct arrived
only yesterday, and you were the first
adventurer I thought of.

Enjoy!

Mei Francis
Exotic Mounts
Dalaran

For the time being, the mount color is not faction specific like previously believed (a red version was found by data miners, and was believed to be the Horde version), as both the Alliance and Horde is given the blue version.

When you have learned how to summon the mount, you will also be rewarded with the Hearthstoned Feats of Strength.

How to get a Hearthsteed of your very own

Interested in adding the [item=98618] to your World of Warcraft mount collection?

Getting your hands on this unique flying mount is simple . . . and fun! Just win three games of Hearthstone in “Play” or “Arena” mode to get the "Mount Up!" Hearthstone quest reward. The next time you log into World of Warcraft, the Hearthsteed will be waiting for you in the nearest mailbox.