Zombified Piglin

A Zombie Pigman is an irregular mob that spawns naturally in the Nether and appears in the Overworld when lightning strikes within 3-4 blocks of a pig, which is a very rare occurrence. 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
Zombie pigmen roam the Nether in packs of 4 - 10 and make grunting and squealing noises, avoiding cliffs and Ghasts. Attacking one zombie pigman will cause the entire group to turn hostile and swarm the player. Pushing a Zombie Pigman off a cliff, as well as hitting them with a deflected Ghast fireball or otherwise causing enviromental damage to them will not cause response. An aggravated group of pigmen is a force to be reckoned with, as each member will deal of damage to the player per sword hit and a player cannot outrun zombie pigmen without sprinting or accessing a location which they cannot jump up to or strike the player with their Sword. This method does not prove efficient, as the pigmen will crowd around and try to get at the player. If a player finds him/herself trapped, it is possible to kill them with a Bow and Arrows or a Sword from this location without being hurt. While in pursuit of the player, they act like pigs, being led by wheat. They will not try to go around objects to get to the player, giving you time to escape before you get caught up to by a Zombie Pigman group.

If lightning strikes within 3-4 blocks of a pig in the overworld, it will turn to a zombie pigman. If the player distances themselves from the hostile group, waits in a safe location, or enters a portal and returns, the zombie pigmen will eventually "forgive" the player and become neutral again. Switching the difficulty to Peaceful will cause them to de-spawn completely and reset their neutrality in other difficulties.

Zombie pigmen are effectively capable of drowning if under water for an extended period of time. When swimming in lava, they collect theoretical vertical momentum, and upon contact with a solid floor, sustain fall damage. This has often lead to the misconception that they can drown in lava. When walking in the Overworld, Zombie Pigmen will spontaneously combust at random times; Fire will engulf them for a split second, but then disappear without appearing to cause them any harm. The brief flicker of flames will also occur if it is struck by lightning a second time.

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.

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[about two minecraft days (if left alone.)]

After the Beta 1.5 update, pigs struck by lightning in the Overworld would transform into zombie pigmen.

Trivia

 * Zombie pigmen are mostly immune to lava and fire, but they can take damage from lava because it does not prevent fall damage like water does. Therefore, if they float in place long enough, they can accumulate enough fall damage that they simply die the moment they touch any surface, including land and the bottom of the lava lake or stream.
 * A direct hit by a Ghast's fireball will cause a Zombie Pigman to run directly towards the Ghast and attempt to attack it, should it survive the explosion This behavior has only been observed if the Ghast is touching the ground, usually occurring when the Ghast spawns under a low ceiling. This is likely because Zombie Pigmen cannot find a path to an airborne Ghast. However, only the Zombie Pigman which was hit by the fireball will attack the Ghast, nearby Zombie Pigmen will not change their behavior.
 * In the Java coding, zombie pigmen are usually titled "pigzombie" or "zombiepig".
 * Despite holding golden swords, zombie pigmen don't drop them when they die, nor do they break although they do occasionally drop gold nuggets (as of 1.9), which can be crafted into ingots to make a sword. Zombie Pigmen are also capable of dealing more damage on Hard difficulty than the player can with a golden sword.
 * About one in two Zombie pigmen will drop a Gold Nugget.
 * Zombie Pigmen can climb up ladders and attack the player while doing so; however, they will not use this to their advantage, and only use a ladder if it is right in front of them.
 * The Zombie Pigman's head's texture is the zombie-like texture with the pigman part laid on top.
 * Prior to Beta 1.8, Zombie pigmen dropped 0-2 Cooked Porkchops.
 * Since Ghast noises travel everywhere, it is a common misconception that zombie pigmen make the high-pitched whines that actually belong to Ghasts.
 * It is possible to use Zombie Pigmen as a shield to block Ghast fireballs, just as you could use neutral or passive mobs to block skeleton arrows in the overworld. This method can also be used to easily obtain a Gold Nugget without ever turning zombie pigmen hostile to you.
 * Zombie's armor has no durability (i.e. cannot be damaged). Two points of armor gives 8% damage reduction from most sources. It means, you need to deal  physical damage to kill a Zombie Pigman.
 * You can hear their sound effects of them being angry far away and even all the way down to the bedrock layer, like thunder.

Bugs

 * 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.