Bedouin Horse Retinue/Warbirth
In the Warbirth mod, these are some of the more powerful units to grace the battlefield on the side of the Arabs, albeit they are now no longer unique, but are rather mercenaries. Still they retain their special ability to heal nearby units, as well as pelt enemies with their salvos, thus making them highly deadly units especially at the end of the game when sustained marches and larger armies are the norm. A handful of these hardened champion cavalry, scattered around your kingdom, can help very much in stiffening resistance to invaders and recouping losses from battle.
If you are playing against the two main factions that can raise them — namely the Arabs and Babylon — remember that these count as ranged cavalry, so a multitude of cheaper missile light infantry, massed up, can decimate their numbers. It is not advisable to hunt them down using cavalry, since these tend to be crack troops — use your Light Cavalry and your Javelin Cavalry to hunt down lighter units, keeping clear of these warriors the best you can.
Deserts are harsh environments to fight in, and the heat and relative paucity of resources meant that for centuries, the nomadic Arabs would seldom muster massive infantry armies for battle, preferring to place emphasis on speed, mobility and surprise. The limitations of technology also meant that it was also unfeasible to send large armies into deserts — an Achaemenid Persian army dispatched by Darius to Egypt disappeared without a trace — and so in many cases, the power of Islamic empires sometimes rested on obtaining an oligarchic consensus from the many smaller tribes and clans which presided over a certain region, such as the taifas of Spain.