Black Khergit Guards were a type of bandit in earlier versions of Mount&Blade. They are of a higher leven than the Black Khergit Horseman, implying the possibility that, should the Horsemen be recruited, they could in time be upgraded to Guards.
Unlike the Dark Knights, the Black Khergit Guards were not completely removed from the game and can easily be enabled again.
The Black Khergits could have been the inspiration for Bannerlord's Karakhergit faction, and this is reinforced by the fact that "kara" means "black" in Turkic languages.