True ‘horns’ are pointed head projections with a proteinous covering around a core of living bone. Many of the things that we call horns are nothing of the sort.
Animals with true horns include cattle, buffalo, horned and antelopes. But the following technically don’t have horns:
Rhinos - no bone core, although, sadly for pedants everywhere there doesn't seem to be a name other than ‘horn’.
Giraffes - ossicones; no proteinous covering.
Elephants - tusks are teeth; similarly narwhals, walruses, and wild boars.
Deer - antlers, distinguished from horns because they fall off annually.
No insects have true horns.
The keratinous part of a horn slides off the bone core, leaving a hollow cone which has been used since antiquity as a musical instrument. It was also used as a drinking vessel that you couldn’t put it down without first drinking its contents. The legendary cornucopia or ‘horn of plenty’ was an attribute of Fortuna. The story was that Zeus was suckled by a goat named Amalthea, and accidentally broke off one of her horns. To make amends, he invested it with the power of giving its possessor whatever he wanted, and returned it to her.
The best use for a unicorn's horn is to adorn a unicorn.
Michelangelo’s statue of Moses follows a common medieval practice of depicting him with horns. This results from a mistake by St Jerome, who translated the Bible into Latin and misconstrued the Hebrew word karen as ‘horn’ instead of its alternative meaning, ‘ray of light’. The passage is Exodus 34:29, and describes Moses descending from Mount Sinai with rays of light coming from his head. Moses was described as ‘horned’ in English Bibles until 1560.
Many gods have been depicted with horns by Egyptians, Hittites, Greeks, Celts, Buddhists, Scandinavians and Native Americans. Kings in Egypt and Mesopotamia wore headdresses with horns to show they were related to gods.
Get someone else to blow your horn and the sound will carry twice as far.
The word ‘horny’ comes, unsurprisingly, from the similarity between an erect penis and an animal's horn.
The larger a dung-beetle’s horn is, the smaller its testicles are.
There are various accounts of humans growing ‘horns’ of the non-bony rhinoceros type. One such concerns Anna Schimper, ‘the horned nun of Felzen’. In 1795, French revolutionary troops attacked her nunnery and evicted the nuns. The shock sent Anna mad and she went to the local asylum, where she spent much of her time banging her head against a table. After some years a horn started to grow from the bump on her forehead. The more the horn grew, the saner she became and by the time she was sporting a thick curved horn (almost obscuring her right eye) she was sane enough to return to the (by now restored) nunnery.
Schimper stayed at the nunnery for many years, eventually becoming abbess. By 1834 her horn had grown to such proportions that it was hard to conceal under her headdress and she had it cut off. She survived this operation and lived for another couple of years, although the horn had started to re-grow.
Indian rhino horn is worth more, pound for pound, than gold.
One of the leading proponents of traditional Chinese medicine in the west was Dr Bernard Reid (1887-1949). Towards the beginning of the 20th century he translated 16th-century Chinese texts and he was quite clear that rhino horn is not an aphrodisiac:
‘[It is used] for typhoid, headache, and feverish colds; for intermittent fevers with delirium. [It is] ashed and taken with water to treat violent vomiting, food poisoning, and overdosage of poisonous drugs.’ He also noted that it was used for maladies such as devil possession, snake poisoning, arthritis, voice loss and rectal bleeding. On no occasion does he mention that it is an aphrodisiac. In some parts of China the misconception is often pointed-to as an example of anti-Sino hysteria.
As the horn is made from keratin, you would be just as well biting your fingernails. Any positive result from taking rhino horn would be due to the placebo effect so it will work equally well for whatever malady you believe it will cure.