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I have submitted a bug report and a ticket about this pet being learned as non-rare qualities (mine was common), but I was told that this was "working as intended." I've asked WHY it's this way, when both here and WoWHead list it at rare base quality and every other pet from the bags is rare, but I've yet to get a response on that front. Figures :/
This pet DOES occasionally show up as a secondary, but is NOT capturable. It will always be of Uncommon quality in these battles, as well.
The only way to actually acquire one for yourself is to buy it.
Just a tip if you're not seeing these or the Twilight Spiders at Sable Ridge: killing the non-battle critters can get them to respawn as battle pets, and they can show up as secondaries to the other battle pets in Sable Ridge.