From Halopedia, the Halo wiki
Having been captured by Yapyap's forces, Jiralhanae are often employed by the diminutive Unggoy for use as battle-mounts. The Jiralhanae are instructed to punch things while the Unggoy throw plasma grenades. Some Jiralhanae are offered upgraded plasma gloves to deal additional damage against enemy forces, though many brutes also strike back against more overly bossy Unggoy.
Do to them being captured by Yapyap, the Brute Riders don't any armor that's worn by Banished Minors, they hardly wear any armor other than wearing "hexagon" armor by their left shoulder and they have a unique helmet they commonly wear, they have a seat attached to their backs in order to be controlled by a Grunt Rider.
Jiralhanae Riders are exclusive to Yapyap the Destroyer and can be built at Raid Camps in place of Jiralhanae Jumpers. Grunt Riders perform well against structures, okay against infantry, poorly against vehicles, and cannot attack aircraft except the Grunt itself dealing little-to-no damage. In Blitz Grunt riders cost 50 energy. Grunt Riders typically outperform both Jump pack Brutes and Goliaths in engagements.
List of appearances