Any Area Unique Drop is equally likely to be obtained after a successful pickpocket.
Each Area Unique Drop is rolled for separately, so it is possible to receive multiple Area Unique Drops from a single action. The chance of receiving an Area Unique drop is tripled if the 95% Thieving Mastery Pool checkpoint is active.
The chance of receiving the unique drop for an NPC is based on a combination of several factors. The unique drop chance for an NPC is included in the tooltip for your Stealth against that NPC.
The unique drop for the Knight is Knight's Defender.