Obtaining Vicious Mounts

Since Battle for Azeroth, you earn a seasonal Vicious mount when you fill up the Season Rewards bar. The Seasons Rewards bar is located in the Rated tab. To track your progress, mouse over the reward in the bottom-right.

The first time you fill up the Season Rewards bar in a season, you will receive the seasonal Vicious mount (currently Vicious War Spider). This mount is immediately added to your collection, you do not receive a saddle. Then the bar resets.

Check this article if your progress bar isn't advancing, or this article if your progress bar reset and you didn't earn a mount.

Customer Support cannot grant mounts from previous seasons.

VICIOUS SADDLES

After you earned the seasonal Vicious mount, every time you fill up your Season Rewards bar you will receive a Vicious Saddle.

You can exchange Vicious Saddles for Vicious mounts of previous seasons at the Combatant Mount Quartermaster. 

Note: Battle for Azeroth mounts are currently not available at the vendor and will be added at a later date.

RATED BATTLEGROUND ACHIEVEMENTS

The Vicious War Steed (A) and Vicious War Wolf (H) are also awarded by the achievement Veteran of the Alliance / Veteran of the Horde which require you to win 75 Rated Battlegrounds. If you don't play RBG, you can buy these mounts with Vicious Saddles too.