I don't usually post here but I have a rules question that I can't seem to find the answer to anywhere else. If a barbarian uses knockback on a mounted target (or if someone uses bull rush in general) and succeeds, does that mean the target is knocked off their mount or is the mount moved back also?
The Mounted Combat rules have long deserved a dedicated rewrite in Pathfinder, in my humble opinion.
"Stay in the Saddle" is the relevant check (DC 5, does not require an action). An imaginative DM might add +1 to the difficulty (house rule) for each 5pts. of damage the rider takes or for each 5pts. the CMD is exceeded by, considering that jousting competitors got unhorsed frequently.
Edit: Oops, I forgot Armor Check Penalties. Well, there ya go.
I would first have the rider make a Ride Check vs. the Combat Maneuver Check.
If the Ride Check fails, that Bull Rush Check is simply matched against the rider's CMD. If the Bull Rush succeeds, the rider is dismounted.
If the Ride Check succeeds, horse and rider are Bull Rushed together, and I'd grant a +2 Size Bonus to the rider's CMD. If the Bull Rush then succeeds, the horse and rider are Bull Rushed together.
It seems like no RAW?
RAW, you can do it. A mount and its rider are two different creatures and it's possible to target either of them with a Bull Rush.
Stay in the Saddle doesn't apply since it's only applicable when you take damage while mounted, your mount rears unexpectedly, or when an ability calls for such a check.
There's also the Unseat feat which allows you to Bull Rush a mounted foe in addition to dealing the damage, and making them prone on top of that.
And if you want to auto-succeed there's the Lance of Jousting which forces an 'impossible' Ride check.
There's also the Unseat feat which allows you to Bull Rush a mounted foe in addition to dealing the damage, and making them prone on top of that.And if you want to auto-succeed there's the Lance of Jousting which forces an 'impossible' Ride check.
The easiest way to deal with an impossible check is to just not roll.