According to the book Punk Rock Blitzkrieg by Ramones drummer Marky Ramone, when Joey Ramone was born in 1951, an underdeveloped "parasitic twin" was attached to his back.
According to Healthline, a parasitic twin is "an identical twin that has stopped developing during gestation, but is physically attached to the fully developing twin. The fully developed twin is also known as the dominant or autosite twin. The parasitic twin never completes development. Rarely, if ever, do parasitic twins have a complete brain or functional heart. They usually don't share organs with the dominant twin. They can't survive on their own."
Joey's parasitic twin, also called a teratoma, was surgically removed and left him with a life-long scar on his back. After learning this, I did a Google image search on "parasitic twin" and some interesting stuff came up, leaving me wondering how much of Joey's twin was attached to him before the surgery.