Horn of the Vicious War Wolf

Source
This item is a reward from the achievements:
 * Veteran of the Horde - Win 75 rated battlegrounds.
 * Veteran of the Horde II - Win 150 rated battlegrounds.
 * Warbringer of the Horde - 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 Warchief Garrosh Hellscream
After completing the achievement you will receive an in-game mail from Garrosh Hellscream which contains the mount:

Mount Journal
At Warchief Hellscream's command, Horde trainers began raising wolves to thrive in the shock and ferocity of sustained combat.