In DragonVale for the iPhone, iPod Touch, and iPad, the first Legendary Dragon to be introduced is named Kairos, The Legendary Dragon of Time.
You can’t breed Kairos, nor can you purchase him directly, you can only summon him, and it’s going to take some work to collect all of the pieces to summon him (unless you’re overloaded on gems you want to spend).
To get Kairos, you first need to purchase his perch called The Perch of Kairos, which costs 750,000 gold and is available at level 16.
Once you have The Perch of Kairos, you’ll need to upgrade it in order to summon Kairos.
In order to upgrade the perch, you’ll need to collect three artifacts named Chronolith, Statue of Kairos, and Time Claw. Each of these artifacts are put together with seven fragments that you can either win at the Dragon Track, or purchase with gems.
It would costs 2,700 gems to purchase every fragment though, so I would only suggest purchasing fragments that you’re having trouble winning.