From Halopedia, the Halo wiki
Unggoy Riders are select Unggoy that ride Jiralhanae into battle. Some Grunt Riders have a tendency to develop an obsession with biofuels, power management, and gladiatorial fights. Other Grunt Riders suffer “accidents” at the fists of the Brutes if they get too bossy. The rest simply hope to survive long enough to get promoted to Locust driver or methane hauler.
Unggoy 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