I believe that the translators had concern with this translation and a note expressing such is found in the margin notes of Isaiah 34:7, where it gives the following note on unicorn:
Or, rhinocerots.
Studying further Isaiah 34:7:
Indeed this is the translation that is opted for in the Douay-Rheims translation of the Vulgate, but the Vulgate literally reads
unicornes.
Finally, the Peshitta reads similarly to the Masorah here:
Masorah reads
[font="Estrangelo (V1.1)"]Mym0r[/font],
r'eimim.
Peshitta reads
[font="Estrangelo (V1.1)"]0myr[/font], appx.
r'eime.
CAL defines
[font="Estrangelo (V1.1)"]Myr[/font] as buffalo.
Lamsa translates this one as unicorns.
For the sake of completeness:
The Septuagint reads adroi - translated by Sir Lancelot Brenton as 'mighty ones.' This reading correlates with the reading of Targum Jonathan, which translates it
[font="Estrangelo (V1.1)"]Nyrbyg[/font].
Many modern translations opt to translate r'eimim as wild oxen.
For a parallel comparison of these versions, please check out verse 7 on this page in my interlinear project:
Isaiah 34 < Cross-Source Interlinear