The word which in the English language Bible is translated as "unicorn" is mentioned in Deuteronomy 33:17, Numbers 23:22 and 24:8; Psalm 22:21, 29:6 and 92:10; and Isaiah 34:7.
Nowhere in these passages is there any suggestion that anything other than a real animal is being described.
So what was the animal described in the Bible as the ‘unicorn’?
The word used in the Hebrew is re’em. This has been translated in various languages as monoceros, unicorns, unicorn, einhorn and eenhorn, all of which mean ‘one horn’.
Archaeology has in fact provided a powerful clue to the likely meaning of re’em. Mesopotamian reliefs have been excavated which show King Assurnasirpal hunting oxen with one horn. The associated texts show that this animal was called rimu. It is thus highly likely that this was the re’em of the Bible, a wild ox.
The real re’em or wild ox was also known as the aurochs (Bos primigenius). This was the original wild bull depicted in, for example, the famous Lascaux (Cro-magnon) cave paintings. This powerful, formidable beast is now extinct.
Today when we think of a "unicorn" we think of something completely different. We think of a mythical creature. Just as we tend to do when Hebrew words that were describing whales, dinosaurs, large snakes and lizards, are translated as "dragons" in the older versions of English language Bibles.
(The word "dinosaur" is a fairly recent word and had not been invented when the King James Bible was being translated from the Greek, Hebrew and Aramaic.)
It’s no big deal though, we can always go back to the Greek, Hebrew or Aramaic and check up on how any given word, phrase or sentence has been translated and we can also see how it has been translated into other languages and in what ways it was used in the original language.
So while the English translators might not have always used the very best word or word definitions might have changed a little over the years, we can easily go back and find out what was really originally meant because of all the language and manuscript resources that are available to serious scholars and Bible students.