Harpoon
Hypersmurf said:
Game mechanics don't really allow for leaving a weapon stuck in someone.
Check out the description for the Harpoon in Sword and Fist. It is a weapon which lodges in the enemy.
"If you inflict damage on your opponent, the harpoon may lodge in the victim if the victim fails a Reflex save against a DC equal to 10 plus the damage inflicted. The harpooned creature moves at half speed and cannot charge or run. If you control the trailing rope by succeeding at an opposed Strength check while holding it, the harpooned creature can only move within the limits that the rope allows. If the harpooned creature attempts to cast a spell, it must succeed at a Concentration check (DC 15) or fail, losing the spell. The harpooned creature can pull the harpoon from its wound if it has two free hands and it takes a full-round action to do so, but in doing so it inflicts damage on itself equal to the initial damage the harpoon caused."
I'd use the same mechanics for the Boar Spear. It lodges in them if they fail a reflex save (DC 10+damage), and if you succeed at an opposed Strength check, the creature can't move forward, and has to extract itself from the spear in order to move backwards (taking damage equal to the inital damage inflicted). If you fail the opposed roll, you lose the spear, and the creature can only move at half speed and can't charge or run until it has removed the spear. The speared creature has to make a Concentration check to cast spells (DC 15), and takes damage when it tries to extract itself from the spear.
This seems a bit overpowered, even for an Exotic Weapon, so you might give it some sort of drawback, such as an extra penalty to hit (besides the -4 for not having an EWP prof. with it) or a penalty to the opposed Strength roll.