The Parasite Twin.

From the blog dedicated to human oddities, J. Tithonus Pednaud presents: The Human Marvel:

On occasion, an embryo will begin to split but will fail to complete the process. A dominant twin will begin to consume the other and manifest it as an extra limb, an entire dangling torso or even as a second head. These twins are very much alive and living off the blood of their hosts like a parasite ...

Hit play or go to Link [YouTube] | The Human Marvel

Previously on Neatorama: Myrtle Corbin, the four-legged woman (also from The Human Marvel)