Cloud Dragon

Breeding
You can get this dragon by breeding a Fire Dragon with a Water Dragon at the Breeding Mountain or Ultra Breeding Tree.

Breeding a Fire Dragon with a Water Dragon may also result in a Blizzard Dragon.

Damage Calculator
To explain, when attacking with Fire, Cloud deals double damage against Water and Ice, half damage against Earth and Fire, and no damage against Metal. When attacking with Water, Cloud deals double damage against Fire and Electric, medium damage against Water and Ice, and no damage against Plant. When defending, Cloud takes double damage from Water and Electric, half damage from Fire and Metal and take no damage from Plant.

Trivia

 * This dragon shares a similar appearance with the Storm Dragon, despite it's lighter body tone and less- obvious "horns".