The Mounted Gunner has been trained specifically to man and defend mounted guns. Due to the strictly specialized functions in the Helghast army, he is the only Helghast type that is allowed to use the mounted machine gun. Though this gives the Gunner a lot of firepower, a machine gun nest can be outflanked, leaving the Gunner exposed.
First Appearance: Visari Palace: Lazar Highway
There are 2 Mounted Gunners as confirmed by the "Killzone 2 Fansite Kit", also known as Unlockable 4 on Killzone.com, however this Mounted Gunner is not mentioned or depicted in the Killzone 2: Collector's Guide to Campaign and Warzone.
You can melee any of the 2 Mounted Gunners for the Safari Hunter trophy.