Republican Army

[Tarbicus]

it was still the case that they were, for some reason, especially suited for this task.

I've read a reference somewhere that the most fleet of foot and agile were especially chosen to ride with a special unit of cavalry, as a special tactic accredited to a centurion during a siege, where the enemy regularly sortied on horseback from the city with great success. The centurion's tactic was very successful. The chosen men would ride behind the rider and, after practicing intensely, developed the skills to leap from the horse and attack the enemy on foot. I can't find the reference, but it's out there somewhere. I'm fairly sure it was in the Republican period.