People can, and do, grow horns. They are called “cutaneous horns.”
From the World Journal of Surgical Oncology:
Cutaneous horn (cornu cutaneum), is a projectile, conical, dense, hyperkeratotic nodule that resembles the horn of an animal. The horn is composed of compacted keratin.
The wonders of the human race are infinite. You can read more at the above link, or at Neurosurgery Online or at the Human Marvels. Apparently, horns occur more frequently among the elderly, especially women. Of people who grow horns, many work outdoors unprotected from the sun. The horns may develop out of resulting lesions.
From the Human Marvels:
The earliest reliable account can be found in the report of German surgeon Fabricius Hildanus. In the late 1500’s he encountered a man with horns protruding from his forehead. Several other cases have been well documented by noted naturalists and medical experts. In his book Anatomicae Institutiones Corporis Humani Dutch naturalist Bartholinus mentions a patient with a horn measuring 12 inches and in 1696 there was a well know case involving an old woman in France who had her amputated 12 inch horn presented to the King. There is also an account from around the same time regarding the extirpation of a horn nearly ten inches in length from the forehead of a woman of eighty-two. Finally, in 1886 the famous dermatologist Jean Baptiste Emile Vidal presented before the Academie de Medecine a twisted horn from the head of a woman. That horn was ten inches long.
And do you know where you can go to see some human horns here in the US of A? Philadelphia!
You can see actual human horns and more at the “disturbingly informative” Mütter Museum. (Their words, not mine.) The Mütter Museum was founded to educate future doctors about anatomy and human medical anomalies. This website says:
The Mütter Museum, located within the stately, late-Georgian halls of the College of Physicians of Philadelphia (CPP), is sometimes disparaged as a “baby-in-a-bottle freak show,” but this label is simplistic.
Indeed. Back on topic, here are some incredible examples of individuals living with horns.
Ma Zhong Nan
Man from Zheng Zhou
Zhao from Zhan Jiang
Abdul Razak of Narasimharajapura
Saleh Talib Saleh of Yemen
Wax cast of Madame Dimanche of France