These dragons are extremely powerful and useful in dragon city battles.
Since
the July 2013 Light/War update, you will need to breed the pure or pure
hybrids together for a chance to get the legendary dragons.
For example:
Pure + Pure Flame
Pure Ice + Pure Dark
Pure Terra + Pure Metal
Pure Electric + Pure Nature
The new breeding time for legendary dragons are 2 days and 6 hours.
The
chance to get a legendary dragon is 1%, so keep trying and don't give
up easily. You also can breed a legendary dragon by breeding two other
legendary dragons. I got my Droconos Dragon by breeding Crystal and
Mirror.