From Halopedia, the Halo wiki
Cannon Fodder are exclusive to Yapyap the Destroyer and can be built at Banished outposts in place of Grunt squads. Cannon Fodder do poorly against infantry, vehicles, aircraft, and structures, relying heavily on their sheer numbers and buffs from Methane Wagons and Goblins. Their benefits are that they are free to produce, as they only accumulate the unit population, and can be produced at any time without loss of resources.
List of appearances