The recently resurrected show Futurama has always contained a lot of science jokes and, basically, nerd humor. As well as the brain slugs, which show up in a number of episodes, there is whole episode about parasites - 'Parasites Lost' -that I have blogged about before. It raises a lot of interesting issues about parasites and their effect on their host.
The brain slugs in Futurama do not make very good parasites (which makes them funnier) because they are so visible, they make their hosts speak in a very obvious monotone, and their influence on their host is a little crude -
Hermes: (monotonous) I suggest we all go to the Brain Slug planet.
Fry: What do we do there?
Hermes: (monotonous) Just walk around, not wearing a helmet.
There are several patterns for knitted or crocheted brain slugs on the internet. The one on the cat above is at Etsy. I rather like the baby hat here, or you can find a free pattern here. Of course you could buy the officially licensed brain slug action figure but where's the fun in that?