A Zombie Pigman is an irregular mob that spawns naturally in the Nether and appears in the Overworld when a pig is struck by lightning, which is a very rare occurence. Like wild wolves, zombie pigmen are not initially hostile, but they will converge on the mob or player as a group to attack them if a single member is attacked. 
Behavior: A Zombie Pigman is an irregular mob that spawns naturally in the Nether and appears in the Overworld when a pig is struck by lightning, which is a very rare occurence. Like wild wolves, zombie pigmen are not initially hostile, but they will converge on the mob or player as a group to attack them if a single member is attacked. 
Combat: To an unprepared or careless player, a charging group of zombie pigmen can mean a swift death. However, fighting zombie pigmen becomes feasible when you can manage which of them become hostile and which remain neutral. Zombie pigmen attack on the basis of 'seeing is believing', meaning that they only become hostile when they see one of their own being attacked by the player. Once this happens, all zombie pigmen within a certain radius (about a dozen blocks) become hostile, and will charge the player IF he is within range. Zombie pigmen will also notice attacks from behind walls. The hostility radius is centered around the zombie pigmen that is attacked, not the player, meaning that you can attack a far-away zombie pigman with a bow without nearby zombie pigmen becoming hostile (although this is not advisable). 
By keeping these rules in mind, it is possible to safely pick off groups of zombie pigmen with a bow, or to kill isolated zombie pigmen, provided that the other members of the group are outside their aggressive range, which is rather short. This can be used to collect Gold Nuggets and Rotten Flesh, despite there being only a small chance of them dropping these items. 
Despite the visual effect of their gold sword they deal no greater damage than a zombie. 
A way to kill them without great difficulty is to create a "booth"--a structure with four walls each three blocks tall, two of them (that must meet at a corner) having gaps at the second block up. A player can mine one block out of the bottom of one of the partial walls and attack a Zombie Pigman, then quickly go back inside and close the way in, then use strike-and-run tactics, running up to the walls with the gaps in them, hitting a Zombie Pigman, then running to the closed corner before it can retaliate, continuing until they are all dead. A bow or stone/iron/diamond sword speeds this process up significantly, the first by allowing the player just to sit in the closed corner and shoot, and the second by reducing the number of hits required. If possible, the booth should be made out of Cobblestone as this will make it invulnerable to Ghasts. 
An easier tactic is to build a column three blocks high. From here, you can either hit the Zombie Pigmen or shoot arrows. They will swarm you, as well as push you around, but you will not receive any damage as long as you remain on the column. 
As of Beta 1.9 Pre-release 5 Zombie Pigmen, Skeletons and Zombies can be damaged by Splash Potions of Healing, and will be healed by Splash Potions of Harming. In addition, a sword enchanted with Smite will do extra damage to Zombie Pigmen. 
Another great way to deal with a pack of charging Zombie Pigmen is to either have a sword enchanted with the knock-back ability, or to backpedal while attacking them, as they can outrun you if you don't sprint. 
Another technique is to agitate them, then lead them into a pool or fountain of lava. This will cause them to return to their docile state. They will not swim around in the lava, only stay in one spot. If you hit them out of the lava from another side, they will return to their hostile state. They will run at you, and fall into the lava, thus calming them again. If there are other pigmen nearby who are also in lava, they will not react. However this is not a good tactic for getting their drops as their drops are burned when they fall into the lava. 
Another way to kill zombie pigmen is to place a bed next to one and right click on it this will make it explode and kill the zombie pigman and other zombie pigmen will not become hostile but it also risks killing yourself because of the explosion. 
History: Zombie pigmen, the first completely neutral, non-passive mob, was introduced in Alpha as a part of the Nether update along with ghasts. The green liquid in the zombie pigman skin was originally blood red, but this was changed because players expressed a preference for the lack of conventional blood and gore in Minecraft. The skin file has "thx xaphobia" written on it as a credit by Notch to the skin's original creator, XaPhobia, also known as X__x.  
Zombie pigmen were originally presented as unarmed passive creatures in a preview before the Halloween update. After Alpha 1.2, their temperament was changed to neutral and they wielded gold swords. Pigmen would never forgive the player for attacking them, but after the Nov. 10, 2010 update, they would revert to a neutral state  after a certain amount of time (if left alone). 
After the Beta 1.5 update, pigs struck by lightning in the Overworld would transform into zombie pigmen.[1] 
After the 1.0 update, Zombie Pigmen no longer drop cooked porkchops upon death, but instead drop rotten flesh. Also, they have a chance to drop Gold Nuggets, which can be crafted into Gold Ingots or Glistering Melons. 
Trivia: If a player attacks a zombie pigman in multiplayer, they turn hostile to ALL players. If the player who attacked them dies or goes away, the zombie pigmen attack the next player who is closest to them. 
In multiplayer, if the server has monsters and a pig is struck by lightning, it may result in a glitched zombie pigman object that behaves similar to the client-side TNT glitch, it can be passed through, does not move or obey physics and cannot be hit. 
In the Beta 1.9 pre-releases, upon looking straight at the Zombie Pigmen with fast graphics turned on, the hotbar seems to change colors. 