I hook his leg...

A combat maneuver I seldom see anyone attempt to catch a foe or their shield to knock them off stance. Here's one way to do it.

Hook Attack
Make a standard attack roll and apply adjustments as below.

-4 if trying to hook an opponent from the front
-2 if trying to hook an opponent from a flank
-2 if using a weapon not specially designed for hooking a foe.
-2 if not using a long weapon.
-1 per leg if foe has more 3 or more legs.

Well made armor still allows AC or armor to be applied. Makeshift shields and shoddy or ill-fitting armor aren't included in a targets AC.

If a hook attack is successful: make a damage roll and compare on chart below.

If a hook attack is made against a foe equipped with a shield they may choose to drop the shield instead of suffering a roll on the following table.

Hook Attack Results
1-3 Foe shaken, they are -1 to any actions they take for remainder of round. no other damage.
4-6 Break Guard, all other attacks agaisnt foe are made at +2 for remainder of round. 1 hp damage is inflicted unless using special capturing weapon.
7-9 Pinned, foe is trapped until pin is broken. They lose DEX bonus to AC . Any bonuses from a defense maneuver are lost. 2 points of damage inflicted unless using special capturing weapon.
10+ Tripped and knocked down, foe falls to ground knocked prone, all penalties as for 7-9 are applied as well. 3 points of damage inflicted unless using special capturing weapon.

If the attack roll was a critical hit an immediate follow-up attack is allowed to the successful attacker along with any damage allowed as per your campaign critical hit rules.

If a hook attack misses: make a save to keep your weapon in hand. I recommend a DEX check with a bonus applied for fighting ability.
If save fails weapon the attacker used is dropped.
If this save is botched (miss by more then 5) : the attacker is off guard, any foes may attack at +2 for remainder of round.
If the save is fumbled (a 1, or a 2 if miss is more then 5): attacker is sent sprawling, they drop weapon and are knocked prone.

Two weapons designed for this maneuver

Crook: A pole designed to catch and hold troublesome animals, it also works well on people.
Damage: 1d4 lethal (or 1d10 when hooking a foe). The weapon is long and requires two hands to use in combat. Cost: 8 s.p.

Spiked Catch-pole: an elaborate polearm with a pseudo-mandible designed to catch troublesome folk.
Damage: 1d4+1 lethal (or 1d12 when hooking a foe). The weapon is long and requires two hands to use in combat. If a foe is pinned or knocked prone the attacker may automatically hit the foe each round for lethal or nonlethal damage. Cost: 30 g.p.

Note on polearms: many but not all polearms are equipped with spikes and hooks that may assist this manuever. If a DM wants to be a stinker or no one is sure if a polearm has an appropriate extension for this maneuver one may be so equipped at a 20% increase to the normal cost of a weapon.