As with all the other Warfront rare mobs, Overseer Krix can only be killed once per Warfront cycle, so this means that you get a shot at the mount roughly once per 12-14 days. His loot resets when your faction captures the Warfront, but in contrast to a lot of the rare mobs in the zone, you can only kill him when your own faction controls the warfront zone. Basically, if there's a portal in your main hub you can kill him and get loot, and if not you have to wait.
So, your faction controls the Warfront zone Arathi Highlands, but where is Overseer Krix located? The answer depends on whether you play Horde or Alliance.
Where to find Overseer Krix
When the Alliance controls the Arathi Highlands, Overseer Krix is located at the Horde's mines.
When the Horde controls the Arathi Highlands, Overseer Krix is located at the Alliance's mines.
The mines are where the opposite faction enters to get Iron for the Warfront Scenario. It should not be too difficult to find him down in the mines, since he chills out in the main entrance room. So just follow the path until you head down the two sets of stairs, and he will be waiting there.