By now, I came with this idea when riding mounstrous mounts.
Both the rider and the mount always use their stats + the rider get +1 AS (+2 with barding) and the mount get +2 AS with barding.
Randomize hits as follows:
1-4 = Mount
5-6 = Rider
If The mount take an injury, you roll as follows:
1-2 = Both the mount and the rider cannot attack until their next recovery phase and the mount is considered as knockdown (not the rider).
3-4 = The mount is stunned. The rider is ejected and roll for a 2'' fall. He can be placed anywhere by the controller in base countact with the mount.If he manage to stay on foot, the rider will not be able to attack this round.
5-6 = The mount is OOA. The rider is ejected and roll for a 2'' fall. He can be placed anywhere by the controller in base countact with the mount. Roll D6, on a roll of 1, the rider has fell under the mount and take an extra automatic hit equal to the mount toughness (without armour save, step aside...). If he manage to stay on foot, the rider will not be able to attack this round.
If the rider take an injury, roll as follows:
1-2 = The rider lose the control of his mount (but stays on it). The mount will attack as normal but cannot be moved, the rider will not be able to attack and is considered as knocked down. If the mount is stupid, it will make his normal test in his next round. Roll a LD test for the rider if they are not in close-combat, if failed, the mount will refuse to move. Make a LD test at each recovery phase to regain control. If failed, the mount continue to stay in place.
3-4 = The rider his stunned. He is ejected and falls 2''. If he survive, place him in base contact with the mount anywhere around it. From now on, the mount count has unridden and behave as his normal habits.
5-6 = The rider is OOA. The mount now count has unridden and behave as his normal habits.
What do you think of it ?