These are (usually) cavalry and appear in many Muslim armies from the the early 9th century starting with the 'Abbasids.
The terminology used for them changed over time. It started out as Ghilman (this is the plural, Ghulam is the singular) which means roughly servant/page or similar with overtones of youth, and then changed to Mamluk (this is the singular, Mamalik is the plural) which means roughly "somebody owned". The latter is a bit of a clue about them as they were originally slaves who were then trained as soldiers, manumitted and were the elite of the armies they served in.
Although some ghilman/mamluk were foot soldiers the terms are used in the MeG lists only for those who fought as cavalry. This has its uses as it means when somebody refers to ghilman or mamluk it is easier to know what they mean.
All these cavalry are armoured horse archers - so they will usually be Protected with Experienced Bow shooting capability. The earliest versions are Flexible Cavalry, but from the later 9th century they are instead classified as Loose Cavalry but gain the Short Spear melee capability. This change is becuase whilst they started off as basically armoured versions of Turkish tribesmen in many ways, they quickly became more adept and inclined to also fight in hand to hand and also use (light) horse armour as well.
As "elite" soldiers they are usually classifed as Superior, and also as a standing army who trained extensively as Drilled. This combination of attributes does make them a bit expensive in points but they can be very effective.
Some later versions may also have Melee Expert as an option, usually "guard" types, and there are some who can have their protection upgraded to ArmHrs/Protected due to having substantial horse armour. There are not many of the latter but they do gain the Shoot & Charge capability and usually have Melee Expert as an option.
Some later mamluks also have Skilled Shooters. Again usually only the "guard" types, however, the Mamluk Egyptian can field them in significant numbers - very effective, but also expensive.
Tips on how to use these troops can be found on the Shooty Cavalry page.
For anyone interested in the development of the ghilman/mamluk "slave soldier" I would recommend two books:
"Slaves on Horses: The Evolution of the Islamic Polity" by Patricia Crone - https://www.amazon.co.uk/Slaves-Horses-Evolution-Islamic-Polity/dp/0521529409/
"The Breaking of a Thousand Swords: A History of the Turkish Military of Samarra (A.H. 200-275/815-889 C.E.)" by Matthew Gordon - https://www.amazon.co.uk/Breaking-Thousand-Swords-Military-H/dp/0791447960
Neither are cheap I'm afraid.
Finally, some images of ghilman/mamulks: