I encountered one near Riften, on my way to castle dawnguard, right after I got the DLC. I believe you have to be at least level 71 before you encounter them randomly, it's just funny that it was the first thing that happened to me after I got the DLC.
There are both Revered dragons and Legendary dragons, different types, the Legendary being the more powerfull.are revered dragons the legendary dragons or are their two new types