Barbarian outposts located near Horses can, in the base game, spawn their own variant of the Barbarian Horse Archer. It was originally a ranged unit, but became a ranged cavalry unit in Rise and Fall.
The Barbarian Horse Archer is a stronger but slower counterpart of the Scythians' Saka Horse Archer. It is treated as a ranged unit, so the attacks of anti-cavalry units such as Spearmen and Pikemen will not do additional damage to it.
As this unit is effectively a modified Saka Horse Archer, all countering tactics for it apply to this unit as well. Outnumber each horse archer to mitigate their range and anti-cavalry immunity with equal or superior units.
We continue to lose our people to the Tatars. They take so many as slaves—tens of thousands—it's as though each year they pluck entire towns from the map. Their horse archers launch lightning raids on our outlying settlements and return with their living spoils. Practically raised in the saddle, these Tatars see themselves as the successors to Genghis Khan himself.
The horse archers pour through our borders, harass our patrols, and fill our militias with arrows before retreating to their khanate. The only walls they fear are those made of pikes.