[The Imp]

The Bear covered most of what I wanted to say. Let me add that we have no idea if anyone has ever warged into a dragon in the past. What we do know is that there is no living creature that a warg has NOT been able to warg into, and powerful wargs are able to control more than one creature at a time. We also know that Varamyr, the most powerful warg we have met (that knows his own powers) thinks Jon is a very powerful warg but doesn't know it (yet). We also know that when asked whether or not dragons can be warged into, GRRM evasively answered "we'll have to wait and see". While it's not a given that dragons can be controlled by wargs, we know that there is a horn that (supposedly) can control dragons, and it would make and interesting story line if one dragon was controlled by the horn and another by a warg, either in cooperation or contest.