It makes little sense to me that the Witch King, in the book, employs both a horse and a fell beast as steeds in the assault on Minas Tirith. Why ride out from Minas Morgul and into battle on a horse when you have a fell beast at your disposal? And how did his fell beast get to the Pelennor? Did it, riderless, accompany the other Nazgul present, or was a small detachment of Orcs and/or Trolls deputised to lead it there? And, when a horse has sufficed for the siege of Minas Tirith and the confrontation with Gandalf, why suddenly switch to the fell beast when the Riders of Rohan pitch up? Wouldn’t that waste time?
To my mind, it seems far more credible that he would choose one steed upon which to ride to war and stick to that.
Jackson’s choice of the fell beast, to my mind, makes sense in the context of the film. While the confrontation between the Witch King on his horse and Gandalf atop Shadowfax at the Gate of Minas Tirith is dramatically compelling in literary terms and has a nice symmetry about it, the Witch King’s appearance on the ramparts to confront Gandalf in the film is visually spectacular (whatever views one might have on the ensuing scene). Similarly, with regard to his encounter with Theoden on the Pelennor Fields. And, of course, it would not have done at all for Eowyn to sever the head of a horse, Mordor steed or not.
