I'm playing a level 69 warrior and I somehow encountered a legendary dragon. I thought they didn't appear in the level'ed list until at least 78? Unless the patch changed something. Was a tough fight, thankfully I had magic resistance gear otherwise its breath attack would have destroyed me.

That's pretty odd...

It might have been the Legendary patch but I don't know.

I'm playing on Legendary difficulty if that helps. I doubt that is the cause though.

I doubt it. Likely some form of glitch. I've never heard of anyone else getting dragons before certain levels but maybe with the new way of leveling in Skyrim the levels don't count for dragons...

I doubt that's the case but still.

It's quite possible that a "hard" or "very hard" spawn point produced a legendary dragon, as spawn points of these difficulties usually produce enemies that are much stronger than you would find elsewhere.