The Ansar Warrior was added in Play the World, and replaces the Knight.
Civilopedia Entry
While early Muslim forces consisted of very few soldiers, it's understood that they retained higher morale and mobility than their enemies, as well as the luxury to retreat into the desert where they alone knew the location of water and grazing land. The one military unit that was present in nearly all of the Arabic expansion of the 7th to 9th centuries was the Ansar Warrior. Directly translated as "Helper of Muhammad", these warriors participated as infantry, but most commonly rode on horseback. The quality of the Arabian horses quickly led to these soldiers dominating the battlefield, making ample use of their array of weaponry, which consisted of javelins, a sword, as well as bow and arrows.