This video shows a news report of a woman in China who has apparently grown a horn at her old age.
Is this a deformity, or some sort of video trick? Is this a scientifically sound phenomenon?
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Tl;dr: Are horns like this known to grow on human heads? Yes. Is this woman's horn in the category known to grow? No idea. Below is documentation demonstrating that these horns do, indeed, grow on people.
One can view Google image results for "cutaneous horn" and see similar formations on humans (though this one is on the larger side).
What I found below was enough to deflate this (for me personally) from the "I've never seen that! This is wild, is it real?" category to the level of "Oh. It seems like this is not that strange at all and apparently scientists have known about this for a long time."
I doubt we'll find a resolution to this particular case. It seems like it hit the news in March 2010 and I don't see any updates. The hits that are out there appear to be regurgitations of one another, such as this, this, and this.
What I can do with an answer is state that cutaneous horns are a real phenomenon. Thus, it very well might be that this woman has sprouted an animal-like horn.
Wikipedia has an article on cutaneous horns which states:
Cutaneous horns, also known by the Latin name cornu cutaneum, are unusual keratinous skin tumors with the appearance of horns, or sometimes of wood or coral. Formally, this is a clinical diagnosis for a "conical projection above the surface of the skin."
Using google scholar I was able to find some other journal articles discussing cutaneous horns.
A histopathological study of 643 cutaneous horns, British Journal of Dermatology.
This paper looks at characteristics that indicate whether the horn was derived from malignant epidermal lesions. It doesn't say much about the horns' characteristics; I'm listing it only to show that documentation exists for at least 643 (what I consider a lot) of these cutaneous horns.
Cutaneous Horns: A Histopathologic Study, Southern Medical Journal:
We report a series of 230 cutaneous horns...
Also quite a few horns, though they could be repeats of the above study.
Gigantic Cutaneous Horns of the Scalp: Lesions With a Gross Similarity to the Horns of Animals: A Report of Four Cases, American Journal of Surgical Pathology (emphasis mine):
Gigantic cutaneous horns, grossly similar to the horns seen in animals, are exceedingly rare in humans. After finding one case in practice, we searched our departmental files for similar cases and examined them grossly and microscopically. Four cases were identified. All occurred as solitary lesions in older women on the parietal–occipital region of the scalp. They had a growth history of up to 30 years; the women hid these horns in their hair. Grossly, the horns were yellow–grey, and there were shallow furrows running along the length of the horns. The length ranged from 17 to 25 cm, and the width was up to 2.5 cm.
Unfortunately, the parietal and occipital lobes are in the back of the head. Let's keep digging...
Cutaneous horns: are these lesions as innocent as they seem to be?, World Journal of Surgical Oncology (emphasis mine):
Cutaneous horns (cornu cutaneum) are uncommon lesions consisting of keratotic material resembling that of an animal horn...
There were 8 male and 3 female patients with a median age of 57 years. Most of the lesions were located on the ear, hand and scalp...
Cutaneous horn is a clinical diagnosis that refers to a conical projection above the surface of the skin. The lesions typically occurs in sun exposed areas, particularly the face, ear, nose, forearms, and dorsum of hands.
Giant Cutaneous Horn: A Patient Report, Annals of Plastic Surgery:
A large cutaneous horn was excised from the left side of the nose and cheek of a 68-year-old woman...
Giant cutaneous horn in an African woman: a case report, Journal of Medical Case Reports:
A cutaneous horn, or cornu cutaneum, is a dense hyperkeratotic conical projection of skin arising from an unusual cohesiveness of keratinized material. It resembles an animal horn grossly, but lacks a bony core, histologically consisting of concentric layers of cornified epithelial cells. Most have a yellow-white color, and may be straight or curved and twisted, and vary from a few millimeters to several centimeters in length.