Susan

Susan the boil is a talking and singing boil on Leela's body. She has always tried to hide it until 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 further evidence of Leela's mutation, similar to how Andy Goldman's mutation presented. This, in addition to her elbow talons, makes for a total of three obvious mutations.

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 as he too is Scottish.
 * She is one among few Futurama female characters to be voiced by a man.
 * Susan's sentience is most likely due to the fact that Leela herself is a mutant.