Porcupine Defense (Combat)Source Dragonslayer's Handbook pg. 25 (Amazon)
You are a threat to opponents that try to overwhelm you.
Prerequisites: Combat Expertise, Combat Reflexes,
base attack bonus +6.
Benefit: Whenever an opponent of at least one
size category larger than yourself attempts a combat
maneuver attack against you, you gain a +2 bonus on
any allowed attacks of opportunity. If your opponent
may make such attacks without provoking attacks
of opportunity, you instead gain a +2 bonus to CMD
against such maneuvers.Source Armor Master's Handbook pg. 21 (Amazon)
You can spend 5 stamina points to make an attack of opportunity against an opponent at least one size category larger than you if that opponent attempts a combat maneuver check against you. You can perform this attack of opportunity even if your opponent can attempt such combat maneuvers without normally provoking attacks of opportunity. (Dragonslayer’s Handbook 25)