These mobs appear all over Pandaria, not just on the new islands, and according to another commenter, Ozlem, the Warbringers are 92 rares which may require a group, or, as the wowhead article asserts, a tank or a healer at the very least. The scouts, conversely, are easier to solo.
Both types of mob, far from being simply an achievement criteria, also drop bags. It seems from what has been discovered so far that the Warbringers can, amongst other things including crafting materials and reputation items, drop one of three mounts. The reputation rewards are bind on account, which naturally means that they can be sent to lower level characters to turn in for reputation with all the current Pandaria factions.
It's not completely clear at this point exactly which mounts they will drop, but it will likely be three of the new mounts being added in patch 5.2, some of which can be seen in action on the preview trailer. This is purely speculation, with just a little hope mixed in, but the only mount in wowhead's list which has three versions is the Primordial Direhorn. There is also other information around that suggests this is the mount dropped, and the warbringers seem, at least some of the time, to be riding these triceratops-esque mounts. Whether or not all three mounts available from these mobs are the Direhorns remains to be seen, but I'll certainly be hunting some rares come 5.2!
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.