I looked it up in the Blue letter Bible and it says...
H7214 (for all of those scriptures...)
- probably the great aurochs or wild bulls which are now extinct. The exact meaning is not known.
Strong’s Definitions [?](Strong’s Definitions Legend)
רְאֵם rᵉʼêm, reh-ame'; or רְאֵים rᵉʼêymlemma רְאיֵם second vowel, corrected to רְאֵים; or רֵים rêym; or רֵם rêm; from
H7213; a wild bull (from its conspicuousness):—unicorn.../
So I guess they don't really know.