There has been mention of Rhonabwy supporting fringe groups that endorse Welsh independence, specifically such as the following of the Pendragon. I forget which book mentioned that, Dragons of the Sixth World IIRC. At the very least, that leads me to believe that its not Rhonabwy masquerading as Pendragon, not to mention that Rhonabwy doesn't seem very inclined to take metahuman form at all. So, if there is an immortal involved, its highly unlikely to be a dragon. If it is an IE, the question would be whether its one we know or a completely unknown one. Then again, considering the typical attitudes between IEs and Dragons, I'm not too sure that Rhonabwy would work with or even support and organization or movement started by one. Not unless he saw a great deal of benefit from it, at least.
EDIT: After going back and reading the section on Rhonabwy in DotSW Game Information section, it does clarify his stance on Elves a bit more. It seems he doesn't really have the same bias against elves that most Great Dragons have, instead he has an issue with regimes or organizations that promote elven superiority or privilege. Guess that opens it up again to having the Pendragon be an IE.