In the sepulcher of the first ones in the immortal hearth area there is a chest that opens if you kill a trash mob with 3 other packs pulled along with it.
As of right now if your character loots a Mawforged bridle from the chest you can loot the chest as many times as you want for more. It vendors for ten gold so it isn't an practical gold making exploit considering it takes a raid group with a reasonable amount of gear to do but it saves repeated clears to farm the item for mount crafting.
As a side note if your character doesn't get a bridle the first time the chest is open it won't appear at all for that week.