[Gaffer]

Remnants of vestigial sibling twin that was not completely absorbed during gestation.

[ought-six]

That would be my guess, as well.

[ArtDodger]

That was my first thought.. although not as succinctly stated.

[ETL]

"Another explanation is a so-called "teratoma" tumor. This type of abnormality, whose Greek name translates to "monster tumor," can grow teeth and hair. In humans, it can grow fingers and toes."

I wonder if it's related to this "horny women" abnormality in humans?