Huldra are forest creatures from Scandinavian folklore, best known for their seductive natures. They are frequently depicted as beautiful women, with the additional traits of a cow-like tail and occasionally a hole in their backs, like that of a rotting log.
As we are starting with a human base, I would expect the addition of a tail to be easy enough to explain, as it is often represented with the same skin tone and hair color as on other parts of the body. The hole in their backs, however, is something far more difficult to justify. The hole is often a true cavity, not unlike one found in a hollow oak tree or rotten log, which brings up questions about the placement of internal organs and other supportive structures.
As one legend states that huldras are unwashed and hidden children of Eve, consider these creatures to be related to or have descended from humans on some level.