There was a bug where anyone who had the Pandaren Monk pet would get the Panda achieve tied to the original Vanilla WoW Collector's Edition Panda pet.
I think I saw somewhere that they fixed it, but idk. I know I still have the achieve even though I never got vanilla WoW Collector's Edition. (And I wouldn't have chosen panda if I had. I would probably have chosen Diablo or zergling pet instead.)