Two new items have been added to the Patch 10.2.7 PTR: Bones of Mannoroth and a Cosmetic version of the Tusks of Mannoroth. The Tusks of Mannoroth is one of the rarest transmogs in the game with the drop chance of this item estimated at around 0.1% to 0.3%.
It has been made slightly easier with a Siege of Orgrimmar Skip in Patch 10.1.5, but at those drop rates, it'll still take a long time to farm.
In the latest Patch 10.2.7 PTR, two new items have been datamined, the first being a cosmetic version of Tusks of Mannoroth.
Tusks of Mannoroth — Use: Add this appearance to your collection. The second item is Bones of Mannoroth which seems to be obtained from defeating Garrosh Hellscream on non-LFR difficulty.
Bones of Mannoroth — Earned by defeating Garrosh Hellscream on Normal or greater difficulty in the Siege of Orgrimmar. While we don't know exactly if these items are connected, their names and being added together does imply that they might.