Numidian Cavalry lack the usual weakness to Spearmen, having a much smaller difference in strength. (Spearmen will still have a minor advantage, but not much.) Like most mounted units, it lacks defensive bonuses, and it has a higher-than-normal retreat chance, so it's better to attack the Spearmen rather than defending against them in many cases.
With bonuses against both Melee and Catapults, Numidian Cavalry can take out many threats on the field, but will be vulnerable to other mounted units.
Numidia was a Berber kingdom located in northern Africa to the west of Carthage. Originally nomadic tribesmen and well-versed in horsemanship, the Numidians were most famous for their cavalry, which was considered to be among the finest in the ancient world. These warriors were lightly-armed - riding on swift horses equipped with nothing more than a small shield and a fistful of javelins - but very tough customers nonetheless. Even after settling down in cities, the Numidians remained enamored with the horse warriors - the Numidians did eventually produce infantry, but it was always regarded as unreliable and substantially inferior to the cavalry.
Numidian cavalry specialized in hit-and-run attacks, making excellent use of their javelins to harass and break up enemy formations. They were never intended to close with the enemy and fight hand-to-hand. Roman commanders were initially skeptical of the threat posed by such lightly armed cavalry, but they soon learned of the danger caused by a force that constantly would attack, retreat and regroup, and then attack again, always remaining out of range of melee weapons. Ancient authors compare the Numidian cavalry to a pack of dogs, because of the way that they savaged the flanks and rear of enemy formations. It required strict discipline for opposing infantry not to break ranks when they came under attack from Numidian cavalry, but in time the Romans learned from their mistakes and became more adept at combating them.
Numidian cavalry was not the fastest, but since the riders wore no armor, they were nimbler than the opposition of the day and able to traverse terrain too difficult for heavier cavalry. Numidian cavalry is most famous for serving in Hannibal's cavalry wing during the Second Punic War. However, they were also employed by the Romans as mercenaries as well. Julius Caesar deployed Numidians during his campaigns - and also fought against them during the Roman Civil War. In short, Numidian cavalry was highly prized and among the best light cavalry in the ancient world.