The Old Republic Guard was a military rank held by the military personnel of the Old Republic.
This term was applied by historians to those member of the Old Republic's army forces that were assigned to guard their naval warships during the time of the Mandalorian Wars. These army troopers were often transported from one location to another but the long transport times as well as the high frequency of space conflicts meant that they often saw action aboard their transport vessels. When their vessels were boarded by the Mandalorians, the members of the Old Republic Guard often defended the ship and attempted to repel the intruders in order to allow the vessel to ultimately reach its destination.
The Armies of the Republic were also made up of the local militias as well as the sector forces who reported to their generals rather then the central command authority such as the Defense Ministry. Darth Revan, before he fell to the Dark side of the Force, was the first field marshal who commanded all the separate units in order for the Republic to better coordinate their defense against the Mandalorian Neo-Crusaders. However, the nature of the conflict meant that they often spent more time as guards on their capital ships then as troopers on the ground.