Information
The Phantom Dragon once known as a cancelled Dragon from Dragon Mounts made it's way into the extra dragons.
The Dragon has 2 varients.
The Old Phantom Dragon
The Phantom Dragon
Both of which hatch from a Phantom Dragon Egg.
How To Get
To get the Phantom egg place a Skeleton egg or Ender Dragon egg on emerald blocks and after 5 minutes will convert into a Phantom egg.
Wait another 20 minutes for it to begin the hatching process and then wait another minute for it to fully hatch to either get the Old Phantom Dragon or Phantom Dragon.