No idea if it is a world first or not, but if pre-cataclysm kills carry over than it is not that hard.
I am on over 20k critter kills personally, and I know people with more.
It's not retroactive. Although I noticed that while grouped with my girlfriend of the same guild, we were getting two critter kills per critter. So maybe if you had a group of 5 guild members or more, it would be 5xcritters killed. Which would work well in maybe Borean, with the million penguins.