These cavalrymen are from the nobility of Numidian society. They are well armoured and can fight confidently as skirmishers or in hand-to-hand combat.
African nobility and aristocracy of regions conquered by foreign tribes or entirely foreign societies sometimes remain in place - occasionally even empowered over local rivals who had sided against the conqueror. These empowered men are of good standing, and are used as local administrators or taken on campaign as a benefit to the local troops they lead. The bodyguards of these men come from the African nobility of the many Numidian tribes and are equipped with good armour, a sword, javelins, shield and a quality helmet. They fight well as long as they are lead by a capable general and will give a hard fight to any enemy because of the long tradition of African skirmishing cavalry.
Historically, African nobles often campaigned for people outside of their culture such as Carthaginians and Romans. This was seen mostly during the Second Punic War when tribes with a common culture chose opposite parties to support. We hear of names like Narawas, Syphax, or Masinissa, who changed from Carthage to Rome and lead indirectly to the victory of the Romans at Zama.