If we want our warbeasts to have armour, it might be good to use bonemail under the skin, as in
the neck of a Stegosaurus. Probably a bit thicker than that, though. It's actually rather similar to how
ankylosaurs used their armour (which would also be a good idea to use). By using armour like this, we'll have a far greater running ability (ankylosaurs were actually pretty good runners, despite their simply enormous girth), and they will basically take papercuts from arrows and small or slashing weapons. Although the force of the stab, as with real chainmail, might still cause some damage.