Reins of the Vicious War Steed

Source
This item is a reward from each of the achievements:
 * Veteran of the Alliance - Win 75 rated battlegrounds.
 * Veteran of the Alliance II - Win 150 rated battlegrounds.
 * Warbound Veteran of the Alliance - Win 300 rated battlegrounds.

Note that this mount is not account-wide, but it can be mailed between characters. Thus, the character that wishes to use the mount must individually learn it.

Additionally, the mount can be purchased for a Vicious Saddle, awarded for completing the Grievous Combatant achievement, which requires for completing 100 Arena (3v3) or 40 Rated Battlegrounds in Season 14.

Letter from King Varian Wrynn
After completing the achievement you will receive an in-game mail from King Varian Wrynn which contains the mount:

Mount Journal
Armored to withstand collisions in battle.