You might have seen the story about the Egyptian girl having a second head removed. The head was part of a parasitic twin which never fully developed and is dependent on the host twin.
You really don’t hear too much about parasitic twins or even conjoined twins much. Mostly these come from 3rd world countries. I suppose in the U.S. and other developed nations, these type of twins would be classified as severly deformed and be terminated through abortion.
Here’s a webpage which has a large number of pictures of parasitic twins.