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, adventurer. 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.