Every so often, Blizzard Entertainment makes some special in-game rewards that are intended to entice players to try more than one Blizzard game. Such is the case for the Cardback Mount in Heroes of the Storm.
The only way to earn this special mount is to play Hearthstone. The mount looks like a card back from the Hearthstone game. If you already play Hearthstone, and have been playing it for a while, there is a good chance that you have already done what you need to earn the Cardback Mount.