The Zombie Pigman is a neutral mob, found naturally in the nether, and in the overworld, when lightning strikes within 3-4 blocks of a pig. This is a rare occurance.
The Zombie Pigman will not attack you, unless you attack them first. But, be careful; if you attack one, the entire mob will come after you. Be careful and wear armour if farming them.
When killed, they drop rotten flesh, and on the occasion, a gold nugget.