Acquire special horse feed used for feeding a spirit horse. Merideth Carlson in Southshore apparently is the source for such food.
Travel to the Dire Maul dungeon in Feralas and slay Tendris Warpwood. Doing so will free the Ancient Equine Spirit. Feed it the special horse feed, thereby soothing the spirit. Finally, give it the Arcanite Barding so it may bless it.
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, <name>.
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.
The Ancient Equine Spirit accepts the offering of the Manna-Enriched Horse Feed, nuzzling the container as it helps itself to the special oats and grains. After it has eaten a little, it looks at the barding you possess and positions itself as though it wants you to place it upon its back...