The Dunlending Warg rider is an Easter Egg that has been part of the mod since Public Beta 26. Very occasionally (originally a 1 in 100,000 chance, now 1 in 10,000), a Dunlending with a bomb for a hat and a second bomb in their hand will spawn on top of a warg. It is unknown what causes this to happen, although Uruk-hai having a laugh is the most popular explanation.
When they see an enemy of Dunland, they will run toward them and explode, much like a Warg Bombardier. They won't die in the blast, and they will also continue to carry a bomb after detonating. They can spawn as a result of conquest, just like ordinary NPCs, and do not despawn.
It is claimed to have been discovered by
Hail_Ceasar, but this is unconfirmed.