In Alfred Bester's book The Stars My Destination, the famous collector of freaks, Harley Baker, brags that he "picked up a genuine fraternal cyst last month". I had no idea what this ment. Nor did E2. Nor did Google. Always a cool thing.
After a bit of research, I'm almost certain that he was referring to a fetus in fetu, that is, a fetus that was absorbed into the body of its twin. These normally (only?) happen in identical twins, as is the case with any type of conjoined twins. Apparently, Baker had found a fetus in fetu, resembling a cyst, that was formed from fraternal twins.