Your barding needs a special form of blessing, one from an appropriate spirit. In Dire Maul is such a spirit, the Ancient Equine Spirit. It is currently held by a corrupt treant, Tendris Warpwood. Pass judgment upon this foul beast, Tortch.
Know this - freedom alone is not enough. You must soothe the spirit before receiving its blessing. For that, special horse feed is needed. Travel to Southshore first and speak with Merideth Carlson on this... and prepare for more sacrifice.
Ancient Equine Spirit: This bridge isn't going to build itself! Now where are those iron pikes and iron rivets?
Ancient Equine Spirit: Great work, Zathus. These supplies will help the bridge effort immensely. We'll get this beauty shored up in no time.