Source & Effect
Argent War Horn is a rare quality trinket bought from an Argent Quartermaster during the Scourge Invasion pre-WotLK event for 40 . It will summon a mounted Argent Knight to fight by your side for 30 seconds before it humorously bubble-hearths to despawn, regardless of what it was previously doing. This effect has a 10 minute cooldown.
The Argent Knight has 2442 health. It will occasionally cast Holy Shock and Crusader Strike, hitting for 80-120 and critting around 160-400 damage.
You can heal your Argent Knight also, without being PVP flagged.
Patch 3.0.2 (2008-10-14): Added