Mania, maven of Hunter pets, has also been looking into the drop rates and drop mobs, and has compiled a handy chart on her blog. Between Mania and the news on Warcraft Pets, it seems we can mostly confirm that the rare raptors (such as, for example, Takk the Leaper) will always drop their pets, while those that drop only off elites have a very rare drop rate more similar to the aforementioned slimes or dragons.
Of course, this is all according to preliminary reports compiled by Mania and Warcraft Pets from player submissions and Wowhead comments, and it is a PTR, so things may change. Still, if you're a hardcore collector, or want to get a jump on the inevitable 100 pet achievement when it comes, or just want your own cute baby raptor to squeeze, you may want to start making raptor hunting plans now. And remember, if your rare pet farming luck is horrid, Breanni will be selling a raptor pet as well.
Patch 3.2 will bring about a new 5, 10, and 25 man instance to WoW, and usher in a new 40-man battleground called the Isle of Conquest. WoW.com will have you covered every step of the way, from extensive PTR coverage through the official live release. Check out WoW.com's Guide to Patch 3.2 for all the latest!