Yes, this is what many suppose (cf. my post above). But Tolkien never said what it was, and personally I am strongly against the opinion that they were Mumakil. This just would be too easy

Myself, I'd add this right below the Bombadil and Balrog Wings problem. If we were given the possibility to ask the Professor, I would ask him this as well (I think he must have something in mind, at least vaguely - either Mumakil or oxes or spell-enslaved ankylosauri or monstrous crossbreed of Fenrir&Sleipnir).