The Fiery Warhorse Mount is a very rare drop from Attumen the Huntsman whom is the very first boss of Karazhan. Despite this boss being a part of a raid instance he is very easily soloable by anyone that is at least level 80. If you're level 85 or above this boss will be a breeze. At level 80 not every class will be able to solo him and at level 85 it will be difficult for some classes to solo him but trust me that it is indeed possible. For those unfamiliar with this encounter here is a list of videos for almost every class soloing this boss.
If you have any trouble with this boss just bring some potions and maybe even a flask. Also keep in mind that you will need at least one other person to be in a raid with you before you are even able to attempt Attumen the Huntsman as it requires a raid to enter Karazhan. If you're level 85 it will only take about 10 minutes to clear the trash before Attumen and take him down. Then you have about a 1 in 100 chance to get the Fiery Warhorse Reins as a mount.
While running Karazhan hold onto all the greens you get and Disenchant them into Arcane Dust and Greater Planar Essences. Also keep all of the Netherweave Cloth that you come across as the Enchanting materials as well as Tailoring materials are always in demand on the Auction House and can net you a nice profit. Also all three of these items are part of my 10k an hour guide, which you can find more information on by following the links below.