David Boswell is absolutely correct - animal names are consistently difficult to translate.
The word under inspection is reim (Hebrew: ראם). The word also comes up in Numbers 23:22:
God brought them out of Egypt; they have the strength of a wild ox. (NIV)
Here are some commentaries.
...It is difficult to say what kind of beast is intended by the
original word. The Septuagint translate the word μονοκερως, the
unicorn, or one-horned animal; the Vulgate, sometimes, unicornus; and
in the text rhinocerotis, by which the rhinoceros, a creature which
has its name from the horn on its nose, is supposed to be meant...The
creature referred to is either the rhinoceros, some varieties of which
have two horns on the nose, or the wild bull, urus, or buffalo; though
some think the beast intended is a species of goat; but the rhinoceros
seems the most likely. There is literally a monoceros, or unicorn,
with one large curled ivory horn growing horizontally out of his
snout; but this is not a land animal, it is the modiodan or nurwal, a
marine animal of the whale kind, a horn of which is now before me,
measuring seven feet four inches; but I believe the rhinoceros is that
intended by the sacred writers.
(Clarke's Commentary on the Bible)
This book deals extensively with the word reim (spelled as re'em), the question of unicorns and many other difficult to translate animals in the Bible (some relevant content is available to read as a free preview on google books).