Ender Dragon

The Ender Dragon (unofficially named "Glydia") is the first official boss mob to appear in Minecraft. It only naturally spawns in The End. This mob uses the dragon model Notch created, but utilizes a texture more reminiscent of an Enderman, being black and scaly with purple eyes.

Strategy
Before entering The End, it is very strongly advised that the player first sleep in a bed near the end portal, in case he/she dies during the battle.

Upon entering The End, a purple "boss health bar" appears at the top of the player's screen. This bar indicates the remaining health of the resident Ender Dragon. Each End dimension houses a single Ender Dragon whose health is periodically charged by nearby Ender Crystals. These crystals can be found at the top of Obsidian pillars which dot the landscape. The healing effect of the Ender Crystals can reverse hard-won damage rather quickly, therefore it is recommended that they be destroyed before the player engages the Ender Dragon (arrows may bounce off silver bits of wings).

Destroying an Ender Crystal while the Ender Dragon is being charged by it (indicated by a series of circles appearing between the Ender Crystal and the Ender Dragon) will cause the Ender Dragon to take additional damage instead of being healed (You can destroy an Ender Crystal by either trying to destroy it as you would a regular block, or by attacking it with a sword, a bow and arrow, etc.) When destroyed, the Ender Crystal explodes and is replaced by a block of Bedrock.

The most common and easiest strategy to defeat the Ender Dragon is to destroy all Ender Crystals before attempting to fight the Ender Dragon, as any damage done to the dragon will be reversed. An advisable strategy for this (since Ender Crystals explode), is to shoot arrows from a distance so the player does not damage themselves, and then climb up the Obsidian towers with a ladder, thus ensuring that if the player falls off the tower due to exploding Ender Crystals, they will be able to grab onto the ladder and not die (although watch out as the Ender Dragon can and will destroy your ladders).

The Ender Dragon will flee when looked at, except when charging the player, but otherwise will fly directly at the player. When the Ender Dragon is charging the player, it is directly facing the player, giving the player a good chance of attacking its head, which, will do extra damage (but if you do decide to attack it, you will have less time to try and avoid its attack). The Ender Dragon will also destroy any block it comes into contact with except Obsidian, End Stone and Bedrock (passing through them instead), making TNT strategies difficult if not impossible to successfully execute. Due to the Ender Dragon's speed and flight, projectiles are favored over melee approaches. The Ender Dragon's immense health and evasive nature necessitate a large ammunition stockpile. Lava is useless, as they are immune to fire damage. Water is also useless, as they take no damage from water and will destroy the block. Knockback enchantments will also do nothing. Snowballs have the potential to stun-lock an Ender Dragon as its recovery time is slower than the firing rate of snowballs. Snowballs can also damage the Ender Dragon, along with other thrown entities, such as Ender Pearls and Eggs. The Ender Dragon will take more damage from a melee attack than a ranged attack, thus waiting for it to charge, sidestepping, and then attacking is a viable tactic. Beds, which explode in The End, can be detonated when the Ender Dragon is nearby, causing it considerable damage. Ender Dragons take the most damage from damage dealt to the head. Hitting other areas of the body will not do as much damage.

Once killed, the Ender Dragon will appear to have beams of light spontaneously erupting from its body. It will then explode, dropping enough experience to bring a player from no experience to level 110 (20000 - 10 drops of 1000 experience, one drop of 10000 experience). If you bring an enchantment table and 30 bookcases with you while killing the Ender dragon, you can set it up after it dies and use it every 50 levels for many level 40+ enchantments. It will also spawn a Dragon Egg on top of an End Portal made of Bedrock. On the second block up, two Torches are placed on either side. Entering the portal will activate a very lengthy (Average of 8 minutes, 30 seconds) text cutscene that acts as the end of Minecraft, after which the player will be teleported to their spawn point in The Overworld. This cutscene can be skipped by pressing the Escape key, and will instantly send you to your spawn point/Bed in The Overworld. The player may return to The End after defeating the Ender Dragon, however there will be no new dragon.

History
Originally, dragons were a possible planned mob, hinted at by Notch during the Spider Jockey update. He also mentioned that dragon lairs may be an addition. They remained a potential work-in-progress until the Ender Dragon was announced.

Notch has stated in the past that if dragons are added, they will not be mountable as it would put too much pressure on SMP servers.

In September 2011, Notch stated that "dragons will be added eventually."

On October 7th 2011, Notch revealed the existence of the Ender Dragon.

On October 10th 2011, Notch released a video showing a small clip of The End that also shows the Ender Dragon fly into the top of the frame. 

On October 11th 2011 A player named stropping_notch asked Jeb if Dragon riding is an idea, to which he replied it is. It is not clear whether this means it is an idea under consideration or not, however, or whether it would apply to the Red Dragons or the Ender Dragon.

On October 12th 2011, in a Reddit post Notch also said, "They will be different dragons. The Ender Dragon will probably become larger, and the ones in the main world will be this size, won't go through terrain, and will be red because dragons are red."

On October 12th 2011, Notch stated in a twitter post that "Dragons have 6 limbs", consisting of 2 wings, and 4 legs.

On October 13th 2011, Beta 1.9 Pre-release 4 was released along with The End. The Ender Dragon existed in the code, mostly functional, however the dragon had not been enabled ingame yet and players were unable to damage it. Certain mods would allow the spawning of the Ender Dragon, but again, no physical interactions were available.

On October 27th 2011, Beta 1.9 Pre-release 5 was released, including the ability for players to damage Ender Dragons. They required mods or a mob-spawner to be brought into the game however.

On November 11 2011, Beta 1.9 Pre-release 6 was released, including a single Ender Dragon as a boss battle, spawning naturally when the player first enters The End.

Bugs

 * If the game crashes while in The End, more Ender Dragons may spawn. This may also happen if Minecraft is closed during the Ender Dragon fight. This can cause problems with leaving The End and the player may have to slay multiple dragons to spawn a portal.
 * Hitting the Ender Dragon with a sword does not lower the sword's durability level (Tested in both SMP & SSP).
 * Sometimes, the Ender Dragon will fly directly at the player without damaging the player, simply pushing them around.
 * Ender Dragons can be damaged by eggs.
 * Slaying the Ender Dragon will not gain you the "Monster Hunter" achievement (although this may be because you can only get the achievement by slaying an aggresive mob, as the Ender Dragon is a boss mob) (Tested in SSP).
 * If you make the Ender Dragon crash into one of the blocks it does not destroy on contact (obsidian, endstone, and bedrock), it will simply vanish, making it impossible to leave The End alive (without cheating).
 * Hitting the Ender Dragons once with any weapons or one of his crystal's explosion will make the player completely immune to the dragon's attack. Restarting the game also seems to correct the bug until you damage it again. (Tested in SMP and SSP).
 * If an Ender Dragon goes far enough from the player, it will stop moving, having a glitchy movement effect, however this is fixed upon getting closer to the Ender Dragon (Tested in SSP). (not really a bug all mobs freeze when more than 40 blocks away)
 * If an Ender Dragon is killed by an exploding ender crystal, it will die while being hit continuously by a healing beam with no source
 * Ender Dragons will still spawn on peaceful mode

Trivia

 * The name "Ender Dragon" was first made known through a tweet by Notch reading "raqreqentba", which could be decoded using the ROT13 cypher, translating to "enderdragon".
 * The Ender Dragon has its own separate mob folder containing separate designs: the old gray skin and the newer black skin.
 * The Ender Dragon flies at 10 times the player's normal walking speed.
 * Made of 61 cuboids, the polycount on the dragon is higher than all models in Minecraft. Notch joked about how Minecraft was "going next gen" when deadmau5 mentioned this.
 * Before it was textured, Notch reported that he was afraid to texture it as it was a complicated model. In the end, the dragon model was so complex Notch stated that he gave up making the texture manually and wrote a texture packer tool to aid him in adding the texture to the model. Notch also uploaded the code for the tool so others could use the pack.
 * As of 1.9 Pre-release 5, Ender Dragons:
 * No longer catch fire from lava or the fire aspect enchantment.
 * Can only be hurt below the 127 block map height limit. Getting Ender Dragons to fly above the limit (in Creative Mode, Overworld) will prevent any damage to them.
 * Phase through the player without causing damage when over block level 127.
 * Have their own achievement called "The End," which requires defeating an Ender Dragon.
 * Ender Dragons also don't seem to be able to take damage or deal damage under block level -10 same as the effect of the player being above block level 127
 * In terms of how difficult the Ender Dragon is to defeat, Notch tweeted that his design guideline for the dragon is " [If] I can beat it at all, it's too easy."
 * Due to the increase in Diamond Armor's durability since Minecraft 1.0.0, Ender Dragon attacks will barely phase the player. However, the launching aspect of their attacks can still subject the player to large amounts of fall damage, as well as possibly knocking the player off of the floating island and into the void.
 * Notch has uploaded a video showing why Ender Dragons will not spawn in the main world, as they would destroy any block they come into contact with.
 * On Reddit, Notch explained why Ender Dragons destroy matter.
 * When an Ender Dragon destroys a block by means of flying through it, an explosion effect occurs identical to that of a TNT explosion.
 * When hit, an Ender Dragon will make the same pain sounds as the player.
 * The knockback caused by an Ender Dragon's attack is far higher than that of any other mob.
 * An Ender Dragon's attack can cause Endermen to chase after it, should they survive the devastating attack.
 * Ender Dragons can be damaged by Snowballs, as snow is made of water, which damages Ender-mobs. However, Snow Golems will not attack it.
 * Ender Dragons do not spawn on the Overworld because if they touch a block besides End Stone, Bedrock, or Obsidian, they will destroy the block, and this can be cataclysmic to maps, making the End an unsafe area to build in until the Ender Dragon is defeated.
 * When no Ender Dragon is present, a player-placed Ender Dragon mob spawner will cause an empty 'boss health' bar to show.
 * Negative splash potions do not seem to affect the Ender Dragon. The potion hits and detonates as per normal, but the dragon does not flash red like when hurt and no apparent effect is seen on it. (Tested in SSP, and with Splash Potions of Poison, Instant Harming, Slowness and Weakness.)
 * The Ender Dragon's hitbox seems to be bigger than the Ender Dragon itself.
 * Even though Ender-mobs are injured by water, the Ender Dragon will destroy water source blocks as if they were solid.
 * If the player uses modifications to spawn an Ender Dragon in the Overworld and uses the portal it creates from its death there, it will lead the player to The End, in a small End Stone room with no portal back to the overworld.
 * The Ender dragon, alongside the Ghast and Blaze, is one of the only current flying mobs in the game. It is the only one which uses wings to fly.
 * Firing large amounts of arrows at the Ender Dragon when it is above the 127 block map height limit will create a "flux", or large amounts of glitched arrows in a spherical pattern, that, when the Ender Dragon descends below the 127 block map height limit, will continually damage it until each arrow has been used. If there are any arrows left in the flux after the Ender Dragon dies, they will shoot out in all directions. If the arrows were launched using a mod item that causes arrows to home in on a target, then the arrows will find a new target to home in on, meaning that if you are in The End, then every enderman there will start to attack you.
 * Playing on Peaceful will not prevent the Ender Dragon from spawning, but it will prevent Endermen from spawning, which makes it easier to destroy the Ender Crystal and attacking the Ender Dragon without distractions.
 * If you load a world with multiple Ender Dragons, there will be a health bar for each Ender Dragon spawned, however most if not all of the health bars are likely to not be seen due to the size of the health bars (see image above).
 * Defeating the Ender Dragon will level you up approximately 110 levels.
 * As long as the Ender Dragon is alive, the space texture of End Portals, as well as the player's view in the End, will have a slight multicolored static overlay to it.
 * If you go beyond the "island" in the End realm, the Ender Dragon will constantly attack you instead of circling around before striking.
 * If the Ender Dragon is killed when there is no solid ground below, the portal to the Overworld will appear approximately where the Ender Dragon died.