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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.