The Matoran Rear Guard Unit was a military unit in the League of Six Kingdoms. Presumably, it was comprised of Matoran that guarded the rear of an army. According to BIONICLE Legends 6: City of the Lost, there were 53 Matoran in a unit.
Shortly before the Barraki were sentenced to the Pit, three members of one of Pridak's Matoran Rear Guard Unit were slacking off, as reported by a battle leader. Pridak ordered the unit and the battle leader destroyed by the rest of the army.