As far as I can tell the units selected for battle animations are indeed largely those who have an existing mob-template. Hence, the oft repackaged Abyssal Soldier/Fiend Lord/Beast of the Pits 'palette-swap' is a frequent sight because it is used for mobs in the Orc Camp, Sea of Flames, Devil's Doorway etc. I have sometimes been surprised by the sight of an exotic unit sword-in-hand, though, so I guess there are some exceptions.
Incidentally, has anyone bought themselves a Priest of the Sands? I'd be eager to know whether the unit appears in battle, considering there are so many versions of the mob in PvE.