Mob

Mobs are the various types of creatures and monsters found in Minecraft. Currently, there are twelve types of mobs. Each of these mobs are affected by the same environmental hazards as the player, including  burning and collision (falling) damage. Up to 200 hostile mobs and 15 neutral mobs can be on a map at a time. Mobs spawn no closer than 24 blocks and disappear if they are more than 4 chunks away from the  player.

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

Mobs
Passive mobs

Passive mobs will not attack the player.

Neutral mob

Neutral mobs will not attack the player unless they are provoked.

Aggressive mobs

Aggressive mobs will attack the player.

Unused mobs

Unused mobs are mobs which are found in the source but are not at use.

Removed mobs

Removed mobs are mobs that once existed in the game but were later removed.

Prefixes
Notch confirmed during the Brocraft.net stream that he will be adding 'prefix mobs'. This meaning that there will be mobs such as 'Burning Spider' 'Giant Zombie' 'Cobble Creeper' and etc. It's unknown how many prefixes there will be.

Pigmen
Aside from Zombie Pigmen, living pigmen have been confirmed to appear in a future update.

Fish
In the halloween update, Notch was planning to add fish, however he had limited time and could only add them as an item (as opposed to a mob). He has said he will add the mob in the future, and in the game files there exists fish coding (though no model).

Others
There are also hints that new mobs might be released in the far future. Below is the list of mobs that have been mentioned by Notch at least once and have a possibility  of appearing in the game.
 * Bats - Notch has briefly mentioned coding bats; however, not much has been mentioned about them since.
 * Dragons - Notch hinted during the Spider-riding skeleton update  that he will add Dragons to the game. Dragons may also be able to fly or  breathe fire.  Notch confirmed in his Twitter that he will definitely  add a form of flying transportation. There is a chance that Dragons may  be able to be ridden in a similar manner to pigs.
 * Wolves - Notch has stated that wolves are going to be a definite addition
 * Whales - Whales have recently been confirmed as a sea mob, although their purpose hasn't been specified. However, the squid has been the update for Beta 1.2, as the first try of whales was right before Beta 1.2, Beta 1.1_02.

Potential Future Mobs
Notch announced on December 20th, that he will potentially add a new mob on or around Christmas. See | here

Notch wrote on December 23rd, around 1 am: Quote "Fixing bugs feels GOOD! And strangely, so does making them." on his Twitter account found here. This may, thanks to the previously announcing of new mobs around christams hint about the new mob/(mobs?) being some kind of insect (or bug). Note: Discussed on the official IRC channel a popular solution was a scorpion.

It is possible Giants may become a future mob, although Giants are still hidden in the code and it is only possible to spawn them with mods or a map editor.
 * Giants - [[Image:Giantzombie.png|left|80px]]Notch briefly ran a test of giant zombies during the very first version of survival (video).


 * Sea Monsters - On July 21st, 2010, Notch commented via Minecast that Sea Monsters are a possibility. He particularly noted the possibility of sharks. Whales have recently been confirmed as a sea mob, although their purpose hasn't been specified. Notch also confirmed on his blog the 3rd of January 2011 that he will add a new water mob.The post. On the 13th of January 2011, Notch added a Squid which is a passive mob that spawns in water. It is unknown if that is the only water mob he is planning to add or if he will add more.
 * Boss Monsters - There is a possibility of boss monsters for the players to slay.
 * Mimics - Notch commented via Minecast that mimics may be a possibility.
 * Friendly/Hostile NPCs - Notch once mentioned having goblin settlements living in NPC Villages.

Trivia

 * The Halloween Update version 1.2.0 allowed hostile mob 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, saying, "it was far too annoying. I have plans on what to do with this."
 * Notch has mentioned ghosts in a recent post which he has removed for the time being, yet may be added back later into the game.

NPCs