Mounted Blade

You can use the momentum of your mount to carry your weapon through one foe and into another.

Prerequisite: Mounted Combat, Ride-By Attack, Ride 3 ranks.

Benefit: When you use the Ride-By Attack feat, if your attack hits, you can also make an attack against a target adjacent to your original target. You take a –5 penalty on this additional attack, which does not benefit from any effects of a charge.

Source: Adventurer's Guide