Obtain a Wolpertinger pet from the Brewfest quest line.
EU players for Brewfest 2012 needed to buy Wolpertinger's Tankard from Belbi Quikswitch for 200 Brewfest Prize Tokens. This may or may not be continued for 2013.
For US players, the quest is Catch the Wild Wolpertinger (A) / (H).
I shouldn't need to write this, but... buying the battle pet off the Auction House will not grant achievement credit.