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 sooner everybody has forgotten 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

 * This character is a parody of Scottish singing sensation who achieved world fame overnight after appearing on  thanks to the internet.
 * This is the among the few female Futurama character to be voiced by a man. Others include The Crushinator and Frida Waterfall.
 * Susan's sentience is most likely due to the fact that Leela herself is a mutant.