With regard to Saucepan man's post, only a few lines before that quote Tolkien refers specifically to the mumakil as "great beasts".
But still, if they really wanted ot have different creatures, why not go for John Howe's version of them, since he was one of the lead artists on the film? I don't have a pic on me, but in a drawing of his of the Siege of Gondor he has these huge things, like twisted great beats of burden (kind of an awesome ox, but without the "cowness" which might otherwise look a bit silly.cute
) - now those are far better creatures imho than our hornéd film-friends.