Mob

Mobs are living, moving game entities. Generally, mobs are affected by the environment in the same ways as the player—they are subject to physics, they can be hurt by catching on fire (except for Witches and Nether mobs such as Zombie Pigmen, Wither Skeletons, Blazes, and Ghasts), falling, drowning, or suffocating, and they can even ride minecarts. When mobs are killed, they drop items which may be useful resources. Mobs come to exist by spawning in various ways. Players can spawn most mobs using spawn eggs in Creative mode. Mobs are aware of players within 16 blocks of them, except for Ghasts, which can see up to 100 blocks away and Endermen, who can see up to 64 blocks away.

The term mob is short for mobile and has been used as a general term referring to any moving entities in games since the first MUDs surfaced.

Current mobs
There are 31 mobs, excluding the player.

Passive
Passive mobs will never attack the player. All passive mobs except squid require land to spawn on. Squids need water to spawn in.

Neutral
Neutral mobs will not attack the player unless provoked. The act of provoking neutral mobs varies.

Tamable
These mobs can be tamed by the player.

Hostile
Hostile mobs will attack the player when in range, each with their own abilities. Cave Spiders, Zombies, Creepers, Skeletons, Spiders, and Spider Jockeys require a light level of 7 or less to naturally spawn. Spiders can become neutral in bright light, but they do not spawn neutral. Hostile mobs do not spawn on "peaceful" difficulty.

Utility
Utility mobs are created by and serve the player. Utility mobs will not hurt the player if created by the latter. An exception is naturally spawned Iron Golems, which can be provoked by attacking the naturally spawned golem or attacking nearby villagers.

Bosses
Boss mobs have more complicated attack patterns and movements. Bosses are meant as end game mobs with their high attack strength and health. When killed, they also drop end game items which can not be obtained in any other way.

Planned

Planned mobs are confirmed to be added in a future version of Minecraft.

Unused

Unused mobs are found in the source, but are not being used at this time. Through third-party modifications, it is still possible to spawn them.

Removed

Removed mobs have been removed from the game and can only be found in previous versions.

Potential future mobs
There are hints that new mobs might be released in the distant future. Below is the list of mobs that have been mentioned at least once by Notch, Jeb, or other Mojang employees who work on Minecraft and have a possibility of appearing in the game.
 * Aside from zombie pigmen, living pigmen have been confirmed to appear in a future update. However, they are not being currently worked on.
 * In the Halloween Update, Notch said he would add fish. However, he had limited time, and was thus only able to add them as an item. There is no coding for fish in the game, and no model was created. Dinnerbone has stated that they have plans for fishing, so it is possible fish mobs will be added to the game.
 * Red dragons have been mentioned in multiple posts by Notch. However, he is no longer working on the game. Jeb, who is currently in charge of Minecraft development, has not announced anything about their current development status.
 * Dinnerbone commented on Reddit that he would like to add pandas to Minecraft.
 * Jeb also confirmed that there will be new mobs in the 1.6 update.

AI
Mobs will ordinarily wander around at random and usually avoid walking off cliffs high enough to cause falling damage (however, there are the rare and amusing sight in which a cow or sheep will walk of a cliff high enough to cause death), though they can be fooled by non-solid blocks such as trapdoors. In this state, mobs (besides Ghasts and Endermen) have a search radius of 16 blocks.

Mobs are capable of climbing ladders just as the player is: if they try to move towards a ladder they're touching they move up it instead. However, mobs do not see ladders nor do they purposely pathfind up them, so they will not attempt to avoid falling off a ladder should they be on the edge of one and they did not climb all walls. Spiders and Cave Spiders can climb all blocks as if they were ladders.

All mobs except slimes will stop wandering within 5 seconds if there is no player within a 32 block radius. In this state, they will glance around randomly, but they won't walk anywhere. They can still be moved by other means such as flowing water, minecarts, etc. Zombies can walk when there is a villager they can attack even if they are over 32 blocks from a player, similarly villagers will flee when there is a zombie chasing them.

Zombies, Skeletons, Creepers and Wolves have a more advanced path finding system compared to other mobs. Here is a video showcasing their path finding system.

Hostile mobs
If the closest player to a wandering hostile mob is within the search radius of 16 blocks and a line of sight can be drawn, (Spiders can always draw line of sight) the mob will enter pursuit mode. In this mode, mobs can see through all blocks and will walk towards the player. Some mobs, most notably skeletons and creepers, will start circling the player (moving to the left from the player's view) when close enough (skeletons at a long range for strafing, Creepers at a very short range). Zombies will sink in water, and whilst in pursuit mode, can path around walls to get to the player. When a hostile mob is attacked by another one, it will attack the attacking mob. For example, when a skeleton arrow hits a zombie, the zombie will attack the skeleton. Also, if a Creeper is killed by a Skeleton's arrow it will drop a music disc, provided that the Creeper does not kill the Skeleton first.

Passive mobs
Animals are attracted to both light and grass. If there is a lot of grass nearby, they will wander toward it regardless of light level. If they are completely surrounded by grass, they will wander aimlessly. If they can't see any grass, they will wander toward light. Passive mobs will flee randomly when attacked. Some passive mobs will follow a player if its currently held item is wheat, seeds, or raw fish. As long as the player does not get too far ahead of the mob, they will follow the player over long distances. This can be used to corral mobs into pens in order to make farms. If wheat is given to the mob by right clicking the mob, the mob will enter breed mode. This does not work with squids. No passive mob, except for squid and ocelots, will despawn if the player moves too far away from them. This also applies to wolves.

Bugs

 * For bugs unique to specific mobs, see that mob's respective bug section.


 * Mobs spin when on fences.

Trivia

 * When a mob walks off a block they will sometimes turn around and get back on as if they didn't mean to get off of it, then walk off yet again and continue on their way. This can be useful when trying to get distance between you and a hostile mob (particularly Creepers)
 * When mobs die they emit puffs of smoke. This gives them the appearance of them turning to dust or vanishing in a puff of smoke.
 * The Halloween Update version 1.2.0 (Alpha) allowed hostile mobs to spawn in higher light levels at lower depths, using the formula 16 - (Layer ÷ 8). At level 8 and below, mobs could spawn even in sunlight. Notch reverted mob spawning to the original method in version 1.2.1 (Alpha) saying, "It was far too annoying. I have plans on what to do with this."
 * Hostile mobs and players can hear up to 16 blocks away. Therefore, if the player hears a hostile mob, it is likely that the mob is aware of the player.
 * Players can approach a hostile mob without being detected if they ensure that there are solid blocks between them and the mob.
 * After killing a player, or if a player dies while hostile mobs are pursuing the player, the hostile mobs will gather around the player's deathplace, attempting to attack the player. This is because the player has been made invisible and non-solid, but still exists in that position. (evident by the "Game Over" screen still remaining in that position). The hostile mobs' heads will "spazz out", a humorous glitch caused by the pathfinding of the mobs so it looks like the hostile mobs are eating the player. A player can use this to their advantage by holding a large number of hostile mobs over their deathplace while another player may pass by unnoticed. This quirk can also work against the player in the case of death by ghast, since (if there is another player present to keep the chunk loaded) the ghast will continue to fire at the player, possibly destroying more of the player's constructions or dropped items. The targeting of the player by the hostile mob is canceled once the player logs out, quits, or respawns.
 * If a hostile mob has found a player and has activated hostile mode, the player can separate him or herself and the mob through a wall, relog, and the mob will no longer be in hostile mode.
 * Mobs still spawn on peaceful, but disappear too quickly for most players to notice.
 * Due to the Skeleton's tendency to shoot instantly on sight, players standing near a skeleton spawner on peaceful may be struck by an arrow during the instant when a skeleton spawns and disappears. This does not always happen but is more evident on a server due to lag.
 * In the Pocket Edition, animals are attracted to water and will return to it if they are pushed out. Also, thy cannot swim in deeper water, and will stay put until removed or slaughtered.
 * On Halloween, 2012, Zombies, Zombie Villagers, Skeletons, Wither Skeletons and Zombie Pigmen had a chance of spawning a pumpkin on their head, with the rare chance of spawning with a Jack-O-Lantern instead.
 * As of snapshot 13w04a mobs will not walk on top of rails unless pushed on to the rails by another mob. This can be used to trap small amounts of mobs, to create a wall quick wall between the player and a mob, or to prevent mobs from interrupting minecarts as they went down rails.