Silly question? Can you 'disarm' a horse, to remove it's rider, ie grab poorly secured item (rider), and leave prone on ground under the horse. I was thinking using diasarm to like steal a clerics holy symbol, or wizards material components bag.
SRD
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Tripping a Mounted Opponent: You may make a trip attack against a mounted opponent. The defender may make a Ride check in place of his Dexterity or Strength check. If you succeed, you pull the rider from his mount.
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Try this instead. Clearly, this works best on low-skill riders. Don't expect to unhorse an expert rider.