*Praefectus socium (sociorum): Roman officer appointed to a command function in an ala sociorum (unit recruited among the socii, Italic peoples of a privileged status within the empire) chosen from the equestrian order.

For some auxiliary troops, specific titles could even refer to their peoples:
* Praefectus Laetorum (Germanic, notably in Gaul)
* Praefectus Sarmatarum gentilium (from the steppes,