3e SRD:Mounted Combat (Feat)
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Mounted Combat [General]
Prerequisite: Ride skill.
Benefit: Once per round when the character's mount is hit in combat, the character may make a Ride check to negate the hit. The hit is negated if the character's Ride check is greater than the attack roll (essentially, the Ride check becomes the mounts Armor Class if its higher than the mounts regular AC).