The Mini Thor is a vanity pet which you get from buying the Collector's Edition of StarCraft II: Wings of Liberty. The pet binds to the battle.net account on which StarCraft is activated, and is given by mail to each and every character you create on that account.
Players are mailed this pet by The WoW Dev Team:
It's time to bring the thunder with your new Mini Thor!
Patch 3.3.5 (2010-06-22): Added.