Ender Dragon

The Ender Dragon is the first official mob boss 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 you first sleep in a bed near the end portal, in case you die 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 who's health is periodically charged by nearby Ender Crystals. These crystals can be found at the summit of Obsidian pillars which dot the otherwise flat 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. Wearing any armor will make the Ender Dragon unable to hurt you; rather, you will simply be pushed around.

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 chunk of Bedrock. Get the Ender Dragon in the explosion to cause even more damage!

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, as mentioned earlier, 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 favoured 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. Snowballs have the potential to stun-lock an Ender Dragon as its recovery time is slower than the firing rate of snowballs. 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 an eight minute text cutscene that acts as the end of Minecraft, after which the player will be teleported to their spawn point in The Overworld. The player may return to The End after defeating the Ender Dragon, however there will be no new dragon. In SMP stepping through the portal will wipe all of your unspent experience levels. There are ways, such as teleportation, to avoid the loss of unspent experience levels.

The enderdragon is immune to water damage, only "exploding" the block, as it would with most other blocks.

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 "[d]ragons have 6 limbs", consisting of 2 wings, 2 legs and 2 arms.

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.

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

 * On SMP, when player is killed by Ender Dragon, the following message appears: *playername* was slain by entity.EnderDragon.name. This is due to the i18n language packs not having values for the EnderDragon.
 * 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 may also cause problems with the Bedrock portal and the player may have to slay multiple dragons to leave The End.
 * 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.
 * Sometimes, there are ~25 ender dragons in the end, you spawn on an obsidian platform in the void, end will not be reset on sleeping, you spawn on any world at the same end, so if you killed all ender crystals, they will also be gone in other worlds and further...
 * Ender Dragons can be damaged by eggs.
 * If you load a world with multiple Ender Dragons, the health bar on top will glitch out (see image).
 * Slaying the Enderdragon will not gain you the "Monster Hunter" achievement. (Tested in SSP)

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.
 * 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, Enderdragon attacks will barely phase the player. However, the launching aspect of their attacks can still subject the player to large amounts of fall damage.
 * 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.
 * Ender Dragons spawn on Peaceful.
 * 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.
 * 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.
 * 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.