Sorry if this is a repost. I'v searched and didn't find it.
Firstly, get an Battle.net Authenticator or a Battle.net Mobile Authenticator (Free with iPad, iPod and iPhone) Enable it from battle.net and log into WoW. You will notice you now have a new pet - Core Hound Pup! He counts for the pet achievements. Now log out again, disable the authenticator and log back in, you will notice you no longer have the pet. Now log out and enable it again. Log in, you get the pet back.. Again, it counts for the pet achievement. Repeat this as long as you want until you get the achievement