James Dean wrote:
Is it an optional rule that lets you target the rider separately?
Yes. A character can choose to target a rider (instead of the rider's horse) only if you are using the Optional Combat Rules (10.D.3.3).
In the basic game, you can only target a rider's horse. Your attack on the horse could, however, hit the rider if an earlier attack in that round kills the horse before your attack is resolved: if your attack would have hit the (now dead) horse, it hits the rider instead.