The Scarred One from the first movie is the dragon whose heart was shared with Rhonu.
A longshot, I know, but it's been confirmed that Draco is one of the dragons in Drago's eggs, so maybe The Scarred One was as well. If they do make a fourth film, maybe they'll explore her story. Besides, from what Drago said about female dragons, Rhonu sounds like she's get along well with the heart of one.- Jossed, In Dragonheart: The Battle for the Heartfire Drago explains that between the events of the third and fourth movie the dragon who’s heart was shared with Rhonu was killed by a lightning strike, causing Rhonu to die as well. This means that the Scarred One could not be the same dragon as she is alive many years after this event.
Gareth and Rhonu are the ancestors of Kara and Lorne is the ancestor of Gilbert.
Because just them being Suspiciously Similiar Substitutes is not enough. Gareth/Rhonu are the ancestors of Queen Aislinn
She is a Celt (same as Rhonu) and is a friend of the dragons who knows about their ability to share their heart with a wounded human.