other translations of the Bible say "wild-ox" instead.

it's worth remembering that the Bible is translated from Hebrew or something.
then Latin, then English from Latin (i could be wrong, but the main idea is that the Bible IS a translation).

it was translated by university scholars, in a time when the world wasn't well-known and words translate in a number of ways, there would be no reason for them not to believe a unicorn was being described and not something similar.

the Bible has many translations, and no one seems to have a problem with the lack of unicorns in most translations. the descriptions are probably mostly (i only saw one) metaphorical in which case the choice of a fictional animal is by no means odd.

julz