To unlock the achievement you have to kill the following bosses from the Killing in the Name of... quest.
These were the original bosses. Once others got added a new achievement got added to cover them. "This achievement was originally called "Pwned All Fur", but got renamed in the 2012 Spring Patch because more Paw and Fur Society bosses were introduced and a new achievement had to be implemented to cover these as well."
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