The rare mobs in Warfront zones such as Arathi Highlands or Darkshore has a very different reset condition than any other rare mob in the game. All the other rare mobs typically tend to reset once per day, or once per week. The Warfront zone rares reset per Warfront cycle instead.
The exact reset happens when your faction gains control of the Warfront zone. This happen after you have been able to queue for the scenario for one week. At this point you will be able to loot every single rare mob once until your faction both loses control of the Warfront zone, and then recaptures it.
This leads to a reset time of roughly 10-14 days depending on how fast the factions can gather resources to begin the Warfront scenario. Since the Warfront scenario is up for 7 days after all the resources has been gathered, it's impossible for the cycle to less than 7 days.
So you might need a bit of patience if you want to farm the Warfront transmog sets or the mount drops by killing rares, since the reset time is so long.
The World boss Ivus also share the same reset rules as all the rare mobs.