Ashlanders in Morrowind are closest you can get in games, having similar aesthetics, though with differences because Vvardenfell. In lore, Bjoulsae Breton tribes are horse nomads. And of course Alik'r though those are Bedouins rather than Mongols. Also there are some on Yokuda i think.
All playable in Elder Kings and EK2 mods for Crusader Kings 2 and 3, though idk how many custom mechanics they have, probably way more in CK2 version since that one had nomad DLC while in CK3 nomads are barebons tribals while the game currently is due for huge DLC byzaboos appeasment.