I'm not sure if "mistranslated" really captures the thought process that went into the King James Bible translation. While aurochs had died out in Britain thousands of years earlier, the legend of the unicorn (probably from Greek knowledge of the Indian rhinoceros) was alive and well throughout the Middle Ages, with distinctly religious symbolism attached.
So, when faced with an unclear Hebrew word for an animal that no English speaker would have ever seen or heard of, and one that represented a wild, horned quadruped, the translators seemingly took the sensible choice of recycling a word that readers and listeners would be familiar with, at the expense of a taxonomic accuracy that was not available to them.
>In common with most other translations of the period, the New Testament was translated from Greek, the Old Testament from Hebrew and Aramaic, and the Apocrypha from Greek and Latin.
>The word “unicorn” appears in the King James Version nine times – in Numbers 23:22 and 24:8, Deuteronomy 33:17, Job 39:9,10, Psalms 22:21, 29:6 and 92:10 and in Isaiah 34:7.
I believe it was from a mix of Greek, Hebrew and a couple of others, mostly depending on which section. I couldn't say which bits these references would have been from. Yeah there's a _lot_ of room for mistranslations or just doing the best they could.
So, when faced with an unclear Hebrew word for an animal that no English speaker would have ever seen or heard of, and one that represented a wild, horned quadruped, the translators seemingly took the sensible choice of recycling a word that readers and listeners would be familiar with, at the expense of a taxonomic accuracy that was not available to them.