Dragon

Dragons are a possible planned mob. Notch hinted during the Spider Jockey update that he might add Dragons to the game. It is also mentioned that dragon lairs may be an addition. There has been no previous work that he has displayed to show that he is currently working on dragons, and they remain a potential work-in-progress until future updates come. It also should be noted that one of the random messages on the title screen is "Absolutely dragon free!". Notch has also stated that achievements could lead into a large cumulative task "like killing a dragon.", although this may be just a typical example. Jeb seems to advise against having dragons as was said at the Dreamhack presentation, at the time 31:34. He also stated that if they are added, they will not be able to be mounts because it would put pressure on servers in SMP.

Notch has stated that dragons will be added "eventually."

It is speculated that Notch is currently working on a new dimension, The Ender, in which Enderdragons will roam. It is currently unknown whether those are the aforementioned dragons or there will be other dragons that do not live in the "Ender". Recently (As of October 9, 2011), Notch has posted some Java code to pastebin, which conatins information about "dragons". (Source)