Instead, you'd find sable riders squatting within a mass of brush, forcing their mounts to crouch while they lay prone atop their backs, waiting for the enemy to get closer, closer, until they spring up, shouting their war songs, far too near for the enemy to flee. Or Bison riders sitting inside a depression in the ground, or a gully, and charging up and out when they spot the foe. Or rhinos behind a hill or hummock, waiting and waiting and waiting. There are enough brush, gulleys, dry riverbeds, sinkholes, ridges, and hummocks in Prax for the old-style ambush to work.
Even if your steed is faster, a mounted ambush works because you're surprised, your bow is unstrung, your steed is just ambling along, your khan has to formulate orders quickly, and shout them out and you have to understand them and then react ... ambushes are bad.
You don't have to be all that close to an enemy to ambush them. For instance, your sable khan, cunning in the ways of Prax, knows that the high llama people in the area will need to use this waterhole for watering their beasts soon. So he stations a war party several kilometers away, in a cluster of skullbushes, or behind a ridge. When his scouts spot the high llamas, the war party waits till they're drinking at the hole, then rides out and around to attack the high llamas from the west. If the high llamas run east, they'll hit a 10 meter deep gully. They can get through it, but it will take time, and by the time they're through (especially herding their beasts), the sables will be upon them. If they run north or south, to get around the gully, the sables can cut across and cut them off. You get the idea.
Also, ambushes work real well if the enemy is not a war party of its own, but has herd beasts, since the herd necessarily runs more slowly than the mounted animals.