Cutaneous horns are hard, conical protrusions from the skin made up of keratin, a strong protein which the body normally produces and is found in skin, hair and nails. It can be soft, as when in the skin, or hard, such as in the nails (hooves are also made of keratin.)
Cutaneous horns are most often found in fair-skinned, elderly patients and develop in areas most exposed to the sun, such as the face, nose, ears, forearms and back of hand, although they can develop on other parts of the body as well.
Cutaneous horns are yellow/brown in color and usually range in size from a few millimeters to a few centimeters. They are curved, which is why they are referred to as horns. They are typically twice as long as the width of the base. While more than one horn can develop, usually there is only one horn.
Possibility of Cancer
The cutaneous horn itself is not cancerous; it is the base of the horn, that may be cancerous, although the majority are benign. Approximately 60 percent are benign, 20 percent are actinic keratosis, which is a precancerous lesion and the remaining 20 percent have bases that are squamous cell or basal cell carcinoma. 
The horn itself is not painful; however, the base can be injured if bumped and may be painful or inflamed from the injury. If the base is painful, there is a greater chance of the lesion being cancerous and should be checked immediately.
Some of the warning signs of cancer include:
Wide base and low height of horn
Redness and inflammation of the base
Diagnosis and Treatment
Because of the possibility of cancer, a cutaneous horn should be evaluated by a dermatologist, who will do a biopsy of the base lesion.
If the horn is found to be cancerous, it may be surgically excised and often some of the tissue surrounding the horn is excised as well to be biopsied to determine the extent of the cancer.
Your doctor may elect to remove the horn in other ways, such as treating with topical chemotherapy agents, frozen with liquid nitrogen or scraped and burned off. Depending on the cancer, radiation treatment may be recommended.
Your doctor will probably recommend follow up care and regular dermatology appointments to prevent or detect skin cancer.
"Actinic Keratosis - Cutaneous Horn," 2011, Feb. 6, Dr. Anthony J. Perri, PerriDermatology.com
"Cutaneous Horn," 2013, June 8, Staff Writer, DermnetNZ.org
"Cutaneous Horn: A Potentially Malignant Entity," 2009, N.F. Fernandes et al, Acta Dermatoven APA
 "Cutaneous Horns: Are These Lesions as Innocent as They Seem to Be?" 2004, Eray Copcu, Nazan Sivrioglu, Nil Culhaci, World Journal of Surgical Oncology