ATTRIBUTES: Make it possible for a player to be knocked from their mount upon taking damage. Assign an attribute that reduces this risk, preferably Agility.
TOOLS: Add snares to the tools category in survival feats, add bolas to the skirmisher category of weapon feats. These items should immobilize mounts, pets, wildlife and thralls for a short duration without causing any damage. Snares would be able to immobilize players, bolas would not. Snare effectiveness/duration scales with the Survival attribute, bolas with Accuracy.
From what I’ve seen in PvP, there’s no real disadvantage for being mounted. Let’s say that at zero points in Agility, the player has a 40% chance of being thrown from their horse when they take damage. Now I have to weigh out the pros and cons of sacrificing Vitality points, for example, to stay on my horse.
Snares would have to a bit heavy and I would add a short animation to setting them up to prevent spamming during combat. You could even add bola or net arrows to a specialist ammo recipe.
Bolas have no effect on player characters because it’s too easy to exploit.
The basic idea is that the player character should have to be skilled at horseback riding in order to be good at mounted combat. On the other side, non-horsed player characters should have to be skilled in the tools necessary to take on mounted opponents while on foot.