Many early-modern Europeans believed in the existence of unicorns, in part, due to the writings of ancient authorities.
Ctesias (ca. 400 BCE), the Greek physician who served the Persian ruler Artaxerxes II, examined the unicorn. In his work entitled On India, Ctesias referenced the unicorn which he called the wild ass of India. It was roughly the size of a horse and had one horn. It also had pharmacological benefits because its horn was an antidote to poison, and those who drank from the horn could not be harmed. The beast could only be killed by bow and javelin. Furthermore, according to Ctesias, unicorn flesh was inedible on account of its bitterness. From roughly 400 BCE onwards, an animal in the shape of a horse with one horn existed in the minds of people, and its horn had the pharmacological benefit of being an antidote to poison.
The Roman scholar Pliny the Elder discussed the unicorn in the eighth book of the Natural History around 77-79 CE. Pliny called the creature “the Licorne or Monoceros,” (monceros in Greek means one-horn, the direct translation into Latin is unicorn). The monceros’ body “resembles a horse, his head a stag, his feet an Elephant, his taile a bore.” It had “one black horne he hath in the mids of his forehead, bearing out two cubits in length.” It was a ferocious beast because “by report, this wild beast cannot possibly be caught aliue.” Livy contributed to the western lore of the unicorn by claiming the unicorn was uncatchable.
The Roman author Aelian/Claudius Aelianus (ca. 175-235 CE) also wrote about the unicorn. His work De natura animalium was a miscellany of facts designed to describe the behavior of animals to illustrate a moral tale. Aelian based his information on mythology, superstition, facts of nature, and mariner’s yarns. In discussing the horn of the Indian ass, which the side notation labeled as “The Horn of the Unicorn,” Aelianus wrote “India produces horses with one horn, they say, and the same country fosters asses with a single horn. And from these horns they make drinking-vessels, and if anyone puts a deadly poison in them and a man drinks, the plot will do him no harm. For it seems that the horn both of the horse and of the ass is antidote to poison.” The notion that unicorn horn had the pharmacological benefit of being an antidote to poison existed in the Greco-Roman tradition for at least 600 years before it was introduced to medieval Europe.
Edward Topsell, The History of Four-Footed Beasts and Serprents (London, 1658).
Ctesias and Andrew Nichols, Ctesias on India and Fragments of his Minor Works (London: Bristol Classical Press, 2011).
Pliny the Elder, The historie of the vvorld: commonly called, the natural historie of C. Plinius Secundus. Translated into English by Philemon Doctor of Physicke, Chapter XXI (London, 1634).
Aelian, On the Characteristics of Animals Book III, section 41, trans. A.F. Sholfield vol. 1 (Cambridge, MA: Harvard University Press, 1958).