Previously the achievement required getting a total of 100 vanity pets, however that total has now been lowered to 75. That is much more accessible for people to achieve, and really requires just a little bit of farming and AH watching. Definitely within the grasp of more people than when it was 100, which would have required more-or-less getting your hands on some super rare pets and/or ebayed TCG loot cards (a subject which some feel is borderline ethical).
Click on the above image to get a better view of the fawn, and start collecting now!
I plan on taking him for a walk through Elwynn Forest when I get him.
For help with tracking your non-combat pet collection, head on over to the most excellent Warcraftpets.com.
Many thanks to Molly for the photo!