Tail antenna
From Tardis Wiki, the free Doctor Who reference
A tail antenna was an antenna which appeared on the rear of K9 models just as real dogs sported tails. (PROSE: Doctor Who and the Invisible Enemy) A robot cat once antagonised by K9 Mark III likewise displayed such an appendage. (COMIC: K9 and Company) One K9 unit, on the other hand, referred to his tail as his rear sensors. (AUDIO: The Choice)