Ender Dragon

The Enderdragon is a new mob that is planned to be included sometime after Notch goes on his third honeymoon, appearing in The End. Jeb has also made it clear that Notch plans to make it very difficult to fight without "enhancements." It will use the Dragon model Notch created, but instead is tinted black with purple eyes and have scales. Dragons will have 6 limbs, consisting of 2 wings, 2 legs and 2 arms. . Not much else is known about them, other than it is one of the "Ender" creatures.

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 Enderdragon was announced.

Notch has, in the past, stated that if Dragons are added, they will not be able to be mounted because it would put pressure on servers in SMP.

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

On October 7th 2011 Notch revealed the existence of the Enderdragon.

On October 10th 2011 Notch released a video showing a small clip of The End which also shows the Enderdragon fly into the top of the frame. 

On October 11th 2011 A player named strap-on-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 Enderdragon.

On October 12th 2011, in a le Reddit post Notch also said, "They will be different dragons. The Enderdragon 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, in a twitter post Notch also said: "Dragons have 6 limbs" Which would consist of 2 wings, 2 legs and 2 arms.

On October 13th 2011, 1.9 Pre-release 4 was released along with The End. The Enderdragon exists mostly functional (with visible gray hitboxes, "borrowed" from Bethesda for debugging) in the coding but it has not been enabled in-game. A community-made mod exist to spawn them by right-clicking while holding a gold sword. They're are also two notably different textures to use, both in a separate "Enderdragon" folder in the Mobs folder.

Trivia

 * The name "Enderdragon" was first made known through a tweet by Notch reading "raqreqentba," which could be decoded using ROT13, translating to "enderdragon."
 * Before it was textured, Notch reported how he was afraid to texture it as it was a complicated model. In the end, the dragon model was so complex Notch reported that he gave up making the texture manually and invented 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.
 * 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.
 * In terms of how difficult the Enderdragon will be to defeat, Notch tweeted that his design guideline for the dragon is "If I can beat it at all, it's too easy."
 * On Reddit, Notch explained why Enderdragons destroy matter.
 * White Stone and Obsidian are the only blocks the Enderdragon cannot destroy, though it can pass through them.
 * According to Notch on Reddit, there will only be one Enderdragon in the End.
 * It is likely that defeating the Enderdragon would be an only possible way to leave End. (The other one is falling in Void, but then you will lose all your items.
 * The Enderdragon may be a boss mob.
 * If a monster spawner is hacked to spawn enderdragons, their parts of body (limbs, torso, head etc) will be locked inside white squares, it will make no attempt to attack a player, but instead will fly towards the closest stronghold and circle above it.
 * On October 12 2011, Notch uploaded a video showing why Enderdragons will not spawn in the main world, as they will destroy any block they come into contact with.
 * Enderdragons cannot pickup blocks like Endermen, instead they destroy them.
 * If you hit an Enderdragon with an arrow, they will deflect the arrow.