Brain slugs
Origin: The Brain Slug Planet
First Appearance: "A Head in the Polls" 2ACV03
General Information
Brain Slugs are little parasitic slugs that attach to an intelligent beings (such as humans) head and control it. They are about as big as a fist, green, translucent and have two feelers and one enormous eye.
Behaviour
A brain slug controls every intelligent being to whose head it is attached. The victim will be likely to speak in a monotone voice and do everything to further the spreading of brain slugs throughout the universe. The recommended procedure of dealing with a brain slug victim is to act casual and switch to a garlic shampoo to avoid being controlled as well. Otherwise assimilation threatens. Ways of denying attachment include wearing a helmet.
Notes
There is a Brain Slug Party on Earth which propagates the spreading of brain slugs, naturally organized by citizens of Earth with brain slugs attached to them. It's political goals include making unreasonably high offerings of money to the Brain Slug Planet and attaching brain slugs to the man on the street.
Two employees of Planet Express have had experience with brain slugs: Hermes in A Head in the Polls and Raging Bender, Fry in Raging Bender. In Fry's case, the brain slug died of starvation shortly after it's attachment to his head. This raises interesting questions about whether the brain slugs rely on the victim's Delta Brain Wave to survive, for Fry lacks this particular wave form. But perhaps it just died because he is an idiot and the brain slug could not sap enough brain power from him.
Judging by the audio commentaries, the brain slugs were initially planned to fulfill a different role. They would have hung on ceilings and dropped on the heads of people, making them smarter for a time, but ultimately covering their entire heads.