Susan

Susan is a talking and singing boil on Turanga Leela's right butt-cheek that was revealed in "Attack of the Killer App". She has always tried to hide it until Philip J. Fry discovered and filmed it. Leela asks Fry to delete the video, but to win a challenge against Bender, Fry sent the video to Twitcher. Soon, everybody knows about the boil, but, just as quickly, everybody forgets the story.

It is possible that this is no ordinary blemish, but another of Leela's mutations, similar to how Andy Goldman's mutation is presented.

Trivia

 * She is a parody of Scottish singing sensation who achieved world fame overnight after her audition  on  went viral on YouTube.  Ferguson was chosen to voice her because he is Scottish.
 * She is one among few Futurama female characters to be voiced by men.
 * Susan's sentience is most likely due to the fact that Leela herself is a mutant.