The Red Dragon is a mob planned by Notch for a future update. It is currently unknown whether Red Dragons will be hostile, passive, or tamable, but it can be guessed that they will either be hostile or tameable mobs. These dragons will not destroy terrain like the Enderdragon, and will be less dangerous. They will likely appear in the Xbox 360 version of Minecraft as well as the original. Red Dragons will probably be quite rare, and if tamable, they would likely be used primarily as a mode of transportation (probably for flying). And, as stated by Jeb, if they were ever to be implemented, you would get them from a dragon egg.