Mob

Mobs are the various types of creatures and monsters found in Minecraft. Currently, there are twelve types of mobs although extra mobs can be implented by mods. Each of these mobs are affected by the same environmental hazards as the player, including burning and collision (falling) damage, with the exception of chickens, who do not suffer collision damage. Mobs can see you up to 16 blocks away, except for the Ghast which can see you 100 blocks away. Up to 200 hostile mobs and 15 neutral mobs can be on a map at a time. Mobs spawn no closer than 24 blocks from the player. Sometimes you will find lots of passive mob at night around a few torches because they spawn in light. This is also a tactic to mine mob drops.

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.

Mobs
Passive

Passive mobs will never attack the player.

Require a light level of 9 or higher to spawn except squids.

Neutral

Neutral mobs will not attack the player unless provoked.

Aggressive

Aggressive mobs will attack the player when in range.

Require a light level of 7 or less to spawn except Ghasts and Slimes.

Unused

Unused mobs are mobs found in the source, but aren't being used at this time.

Removed

Removed mobs are mobs that have been removed from the game.

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).

Dog
Notch confirmed on twitter that he "promised...Dog pets..., so I guess that's confirmed now." With the release of dogs, wolves may also be added for players to tame into dogs. On 15. March 2011, Jeb said on his Twitter that he was "Trying to create the wolf/dog mob, but it either keeps looking like a baby bear or some kind of freak sheep."

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 and that they can be tameable to provide pet dogs . On 15. March 2011, Jeb said on his Twitter that he was "Trying to create the wolf/dog mob, but it either keeps looking like a baby bear or some kind of freak sheep." . [[Media:Wolves_in_working.png|He also uploaded a screenshot of his attempt]]
 * 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 wrote on December 23, around 1 am: Quote "Fixing bugs feels GOOD! And strangely, so does making them." on his Twitter account. This may hint about the new mob(s) being some kind of insect (or bug).

Giants have the zombie skin instead of the normal character's skin. 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 will hurt the player, but only if the player walks into the giant's body, otherwise it will simply push the player. Giants have 100 health.
 * Giants - [[Image:Giantzombie.png|left|80px]]Notch briefly ran a test of giant zombies during the very first version of survival.

On the 13 of January 2011, Notch added Squids, a passive mob that spawns in water. It is likely that more water mobs will be introduced, as Notch stated on his blog that more water content was incoming.
 * Sea Monsters - On July 21, 2010, Notch commented via Minecast that Sea Monsters are a possibility. He particularly noted the possibility of sharks, but has recently stated that he would not be releasing sharks as it would be too difficult for the players to deal with. 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.
 * Boss Monsters - There is a possibility of boss monsters for the players to slay.
 * Mixes A Topic On Minecraft Fourm Has Started To Think about Mixes Of mobs Like Spiderpig Undead Cow etc
 * 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 (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."
 * Mobs will frequently spawn in groups with mobs of the same variety.
 * Mobs will not spawn on transparent blocks.
 * Some animals can climb ladders, and Spiders can climb walls.
 * If a hostile mob doesn't see you, and you hit it with a projectile, the mob will head in your direction, usually finding you.