At 1st level, a warlock gains the Otherworldly Patron feature. The following options are available to a warlock, in addition to those offered in the Player's Handbook:

The Ancient Dragon

You have sworn a pact to a dragon. While most dragons are not seen as those who would seek out warlocks, ancient dragons or great wyrms will sometimes seek out bargains with mortals to extend their region of control over the world. Tiamat and sometimes even Bahamot will also seek outmore personal pacts with their mortal followers

Dragons are universally creatures of greed, and all hunger to fill their hoards, often using their warlocks to seek out wealth, secrets or artifacts to bring back to their layers. Other dragons might beseech mortals to aid them in the evergrowing war between the chromatics and the metallics.

Warlocks who swear pacts to dragons have most likely met their patron in person rather then in vision or dream. Most being kobolds or dragonborn or some other draconic blooded being. But all beings can align themselves with the goals of these ancient godly beings.