The unicorn's religious career rests on a translation decision, and it is a good example of how much a single word can do.
The Hebrew Bible refers several times to an animal called the re'em, in contexts emphasising its strength and its untameable horns, Job asks whether the re'em will consent to serve, and Psalms and Numbers invoke its horns as an image of power. The animal is now generally identified as the aurochs, the enormous wild cattle of Eurasia, hunted to extinction in the seventeenth century.
The translators of the Septuagint, rendering the Hebrew into Greek in Egypt from the third century BCE, gave it as monokeros, one-horned. The reason is not recorded. It may reflect the standard side-on depiction of a horned bovine in ancient art, in which one horn hides the other. The Vulgate followed with unicornis, and the King James Version of 1611 followed the Vulgate, so English readers for three centuries encountered unicorns in scripture.
The consequence was substantial. An animal that would otherwise have remained a curiosity of Greek travel writing became a creature vouched for by God, and could not simply be dismissed. Medieval and early modern naturalists who doubted its existence had scripture to contend with, which is a large part of why the unicorn remained in serious natural history long after comparable creatures had been dropped.
Modern translations render re'em as wild ox and the unicorn has quietly left most Bibles.