I was going through a few zones today, try to capture the unique pet for that zone, and I noticed something. Each unique pet that I battled today was of Poor quality. For example, here are the ones that I captured this afternoon:
Festering Maggot in Eastern Plaguelands Larva and Spirit Crab in Ghostlands Lofty Libram, Snowshoe Hare, and Infested Bear Cub in Hillsbrad Foothills
Each one of these that I battled was Poor. Now I won't exclude the possibility that I was unlucky, but I tried about 5 battles for each one of the above and every one was Poor. Maybe this was just some weird, post-patch action.
Blizzard's RNG has a lot to be desired for pet battles. What I've noticed is that pets that spawn simultaneously will have the same quality. This is why (for instance) when you see a Civet with pups all the pups are of the same quality and have identical adds.
At server up, everything spawns nearly simultaneously. If there's any kind of staggering of initial spawning, then you'll get different qualities.
Once you've cleared stuff out, and then start seeing respawns, you'll see different qualities.