So yeah -- either an insane dragon/Great, or a Horror-tainted one. The difference of which is ... purely academic. 
Trust me, it really isn't. An insane great dragon could do some serious damage before being put down, sure. But the last Horror-possessed dragon that I'm aware of could poison the soil and kill crops just by flying overhead.
There are three possibilities that I can think of from Earthdawn for our dragon escapee:
1. Vestrivan, the aforementioned Horror-possessed dragon, also known as the Despoiler of the Land. Has a bit of a split personality thing going on.
2. One of the dragons that was corrupted by Verjigorm in an astral cocoon (possibly during metamorphasis from adolescence to adulthood).
3. Denairastas, the Outcast. A dragon exiled for interbreeding with humans (which was banned after Alamaise's little oopsie with the Immortal Elves) to raise an army of half-dragon mages to conquer the world. If the IEs are still around in SR, then his children, the Clan of Iopos, could easily be as well.