I may be wrong, but as far as I've been able to tell, pet collecting achievements are based on unique pets we've had, not necessarily pets we currently have.
For example, last night I went out and collected pets to finally get my Littlest Pet Shop achievement. Since I don't have much space (still haven't weeded out all my doubles yet) I released most of the pets I captured. I noticed that when I released pets my progress toward the next achievement, Going to Need More Leashes, didn't go down.
So, the question is, is there an addon that will track pets that we've had at one point but no longer have? I don't feel the need to keep some of these pets but I also don't want to have to keep a written ledger full of the pets that I've already had once and don't need to capture again.
I wish I knew how to make addons. I'd make this one myself.