Two new items were added called 'Vicious Saddle' and 'Vicious War Saddle' and they appear to be the currency for the new mounts. But not only that, but the previous two RBG mounts also require this saddle to purchase. I am very happy with these vendors, as it means they will not be retired in the future or after this season.
Anyway, my theory is that when getting the appropriate achievement after 5.4, you no longer automatically get the Vicious War Steed(http://www.wowhead.com/item=70909) when Alliance or Vicious War Wolf(http://www.wowhead.com/item=70910) when Horde but instead this 'Vicious War Saddle' item that allows you to buy one of two mounts (depending on faction)
Vicious Skeletal Warhorse
Vicious War Wolf
Vicious War Steed
This also means they may add more RBG mounts in the future, more like the black war mounts in Vanilla.
Alliance version of the vendor doesn't have the mounts yet.