In DragonVale for the Android & iOS devices (iPhone, iPad, etc.), the Terradiem Dragon is a limited dragon that represents harmony, and likely coincides with Earth Day. The Terradiem Dragon first available on April 18th 2013 and available to purchase and breed for about one week.
The Terradiem Dragon has a base breeding time of 24 hours, sells for 1,200,000 coins, and earns 200,000 XP for successful breeding once you reach level 14.
To get a Terradiem Dragon, breed dragons with a total of Air, Earth, and Water elements. Mud and Air, or River and Air are good combinations.