It looks as though the collection achievements such as the Littlest Pet Shop are missing on the beta. I can't seem to find them in any section of the achievement window. What this means, I haven't the slightest clue.
One possibility is that it's being relocated into the new Pet Battles tab instead.
I don't think they'll remove the achievements entirely, as it would not only be a real shame but new collectors will lose their chance at the pet rewards from those achievements. But at the same time, with the Pet Journal and account wide collections being introduced in MoP, a collecting achievement is kind of pointless, isn't it?
Then again, if both the Pet Journal and individual spellbook companion collections are maintained, you could technically still track the number of pets for each individual character (since the Pet Journal and the spellbook are two separate things).
It's still early on in the beta, so we'll have to wait and see what Blizzard decides to do.
PS: And no, this is not an April Fool's joke heh.