Zombified Piglin

A Zombie Pigman, or PigZombie, is a neutral mob that spawns naturally in the Nether and appears in the Overworld when a pig is struck by lightning. Like untamed 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 making grunting and squealing noises, avoiding cliffs and Ghasts. Attacking one zombie pigman will cause the entire group to turn hostile and swarm the player. 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.

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 or in contact with lava for an extended period of time. 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
Because they are well-armed and have a horde mentality, zombie pigmen are incredibly dangerous to the unprepared player, especially given the presence of ghasts and the treacherous terrain of the Nether. Those attempting to flee from a few angry pigmen can find themselves cut down by a multitude of their brethren converging on them from all corners on the path in front of them.

Since it is virtually impossible to engage a single zombie  pigman without several more joining the fray, a player should have a full set of armor, a sword (preferably diamond), a bow, plenty of arrows, and (If you're playing 1.8, then the food you get doesn't matter as food doesn't directly affect hp anymore)several pieces of highly nourishing food (cake, mushroom soup, or cooked porkchops) before heading out. Once in the Nether, they can then dig a tunnel into the side of a mountain or erect a small shelter with a single block window to ensure that the zombie pigmen can only approach from one direction. The player can then shoot a single pigman, retreat to safety, and fire upon the approaching horde at their leisure or using the sword at closer quarters. Caution should be taken when exiting the safe area, as there may still be aggravated pigmen in the vicinity. If the player is overwhelmed by a group of zombie pigmen and returns to the Nether soon afterwards, the group will not de-spawn and remain hostile towards them.

Another alternative is to place a sand block and two cactuses on top, then get on top of it and shoot a single zombie pigman. Any nearby will come running at you and kill themselves on the cactus.

History
Zombie pigmen, the first completely neutral, non-passive mob, were 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.

After the Beta 1.9 Pre-release update, zombie pigmen no longer drop cooked porkchops and instead they drop rotten flesh. Also in the 1.9 Pre-release they have a chance to drop Gold Nuggets, which can be crafted into Gold Ingots.

Trivia

 * Zombie pigmen are mostly immune to lava and fire. However, they can take damage from lava as it does not prevent falling damage like water does. Therefore, if they float in place long enough, they can accumulate enough falling damage that they simply die the moment they touch any surface, including land and the bottom of the 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. 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. Zombie Pigmen are also capable, on Hard difficulty, of dealing more damage than the player can with a golden sword.

Bugs

 * If a player attacks a zombie pigman on SMP, 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 SMP, if the server has monsters=false 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.