Parasitic twin

A parasitic twin is a rare biological condition in which a second embryo begins developing in utero, but never fully separates into a twin at birth. Unlike conjoined twins, the secondary fetus has a stunted developmental cycle with the primary twin serving as the dominant form. Or, in other words: It's a whole big bag of Nasty AF. Historically, parasitic twins occur most commonly in India.

In the 1990s, there was a carnival performer who harbored a parasitic twin.

Films

 * Dark Half, The
 * Hellboy II
 * Total Recall

Television

 * The X-Files: Humbug

Novels

 * Dark Half, The