Sarge's Tale. mount is from a Hearthstone promotion, to get it you have to complete the Mercenaries prologue and begin one new Bounty of their own.
This Service includes:
1. Complete the Mercenaries tutorial in Hearthstone
2. Mount: Sarge's Tale.
This cute rat is free when you try out the Mercenararies game mode in Blizzard's card game, Hearthstone. You need to complete the Mercenaries tutorial in Hearthstone and then start your first bounty to automatically unlock Sarge on your World of Warcraft account.
*Mount not available in World of Warcraft Classic or Burning Crusade Classic.