Rear sensor
From Tardis Wiki, the free Doctor Who reference
K9's "tail" was formally his rear sensors.
K9's rear sensors were mildly damaged by a shot from a warrior-bird while on Jokastarna; when K9 complained of it, the Mistress recognised that "they'd bent [his] tail". She straightened it out manually, restoring functionality to the sensors, and allowing K9 to determine that the warrior-birds were not part of Kebek's fleet. (AUDIO: The Choice)
