Mirror Dragon

Breeding
You can get a Legendary Dragon by breeding two Rare Hybrid dragons with each other at the Breeding Mountain or Ultra Breeding Tree. Some examples are:
 * Cool Fire Dragon
 * Soccer Dragon
 * Armadillo Dragon
 * Gummy Dragon
 * Poo Dragon

You can also get the Mirror Dragon with 7 Guesses (20-24 September, 2016

Mirror Gallery
mirror egg

Damage Calculator
To explain, when attacking with Legendary, Mirror deals double damage against other Legendary Dragons, and normal damage against everything else (?). When defending, Mirror takes double damage from Legendary Attacks and half damage from Physical Attacks.