Hero of the Storm

Rewarded for getting [item=122477], from the Heros of the Storm promotion.

This is obtained by reaching player level 20 in Heroes of the Storm.

Blizzard has not stated when, or even if, this promotion will end. So this pet should still be obtainable until stated otherwise.

What that promotion is is still a mystery at this point.

Possible theories at the moment:
Participating in the Alpha?
Participating in the Beta?
Reach a certain rank or level in Heros of the Storm?
Playing X games?
Winning X games? (similar to the Hearthsteed promotion from Hearthsone)
Purchase a specific character, skin, or mount from the Heros of the Storm store? (unlikely)

The pet is a Grave Golem, which is from the Haunted Mines map in Heros of the Storm.

"In one invasion of the Haunted Mines, a team of heroes only managed to collect a single skull. The golem that resulted, too small to do much damage to the enemy towers, wandered off into the Nexus, eventually finding its way to Azeroth."

I wasn't sure if this was still active and wanted [npc=91226]

Just received this achievement today 7.30.2015. Received the pet in the mail. No expert playing HotS and took me about 20 hours of play to get to level 20. I also used a "stimpack" which gives an XP bonus. Currently Blizz is giving a 7 day pack to new players.

@jono1001, yes you can still get the pet and the feat of strength. I got it yesterday 24/2/17, soon as I hit level 20 in HoTS. Didn't get the pet in the mail though, it showed up as an un-opened package in my pet journal.