The classical representation of a werewolf is a generally more or less human looking individual. The only difference in appearance between a werewolf and a human is a excess of hair growing from all over the body. There is an actual medical ailment that includes this particular symptom. This disease is known as Ambras syndrome.
Ambras syndrome, also known as generalized congenital hypertrichosis, is a relatively rare disorder that today is documented in less than 100 individuals worldwide. It manifests itself by having hair grow all over the body, not just the head. It is thought that a mutation on chromosome 17 has lead to the addition of this mutation in the individuals affected by the syndrome.
This particular disorder has caused many sensational sideshows over the centuries. Humanity has always been fascinated by the bizarre and this seemingly more animal human has always drawn a crowd. One of the most famous sideshow performers with this disorder toured with Barnum and Bailey and was known as Jojo the Dog faced boy.
In medicine there are two kinds of excessive hair disorders. There is hirsutism which commonly occurs in women. This is due to the presence of androgens that induce the growth of hair like that in males. This disease could signal anything from polycystic ovary syndrome to tumors on the adrenal cortex. Ambras syndrome on the other hand is non-androgen related and is a byproduct of some genetic disorders and a rare side effect in some medicines.
The fascination with this particular disorder started during the middle ages especially when particular infant from the Canary Islands that was afflicted with the disease was presented to King Henry II in Paris. Petrus Gonsalvus/ Gonzales came to Paris mainly as a oddity for the nobility. Considered initially as a savage and thus inept to learn by the King. He not only proved the king wrong but became quite the expert on etiquette and languages. Later he married and had at the very least two documented children with the same disorder as himself.
Since the discovery of Petrus and his family less than 34 cases of Ambras syndrome have been authenticated to the medical community. Yet the multiple legends of werewolves have persisted across the many centuries. This could at the very least be of a very early run in with an afflicted individual that has Ambras. The hairiness of these individual leads to them being classified as something else, thus leading to at the least the physical appearance of a werewolf, of course this was not helped by the wearing of animal skins into battle by the Nordic Berserker.
List of Sources
Acquired generalized hypertrichosis article
Schloss Ambras article
Petrus Gonzales article
Petrus Gonsalvus article
Congential Hypertrichosis Lanuginosa article