Have the same idea but don't think wargs should have any knockback. They're probably not heavily built enough for that.
Think horses(not ponies, or zebras) similar to should briefly stun targets at full charge. Probably being design in a way that a Rohirrim horse rider will consistently take out an Uruk in open fields.
To balance it out it should take a notable amount of time to start a charge. Probably "longer" as the current rhino charges(which are pretty ridiculous you can start charge right next to someone they should charge that). So that notable distance is needed to do this. And striking the horse can cancel the charge(imagine pikes would be best for this). Should also state more heavily armored horses should have more pain resistances(take more hits to stop their charge) but less controllable when charging. Probably Rohan horses in general should have more pain resistance.