Swift Horde Wolf

As of patch 4.0.3a, Argent Crusade Tribute Chest no longer spawns so that mount is no longer obtainable.

Loot from 10 man Heroic Trial of the Grand Crusader with all 50 attempts left, only one drops per raid.

This is a more "Horde" themed mount with a little more armor plating than your regular wolf. It also has a symbol of the Horde branded on it's back thigh. It's the perfect mount to represent your faction on as you're riding in Battlegrounds. Here's a picture of it from MMO-Champion:

http://static.mmo-champion.com/mmoc/images/news/2009/july/swifthordewolf.jpg

Edit: It seems this is just the mount you ride as part of the Trial of the Crusader dungeon. This is the 5-man dungeon encounter of the Coliseum. If you look closely at the mounts you'll notice it has the same skin. There still remains the possibility that this is some sort of reward for an unlocked achievement, but I don't hold a lot of faith in that.

Edit: Confirmed. This mount is a reward for doing 10-man Trial of the Crusader without any wipes. Very classy mount that will show off your love for the Horde.