Actually was just going through it a moment ago and realized something. The achievement for 150 pets might not be available for people who hasn't paid money for pets in one way or other (Blizzard Store pets, TCG pets, Collector's Edition boxes).
If this is true, that is a sad, sad fact and worth frowning upon Blizzard for >
I hope I'm wrong or they plan to add enough pets in the game so anyone can reach this without paying real life currency for it ("anyone" who has mad patience camping rare mobs or paying silly amount of gold for rare world drops).