Would it be possible for a Bjornaer’s Inner Heartbeast to be a mythic version of their outer Heartbeast? Not a chimera, but a long extinct or legendary creature, like a sabertooth lion or unicorn? Or an Archelon, the largest sea turtle in history? (Sure, increased size can be accomplished, but not a change to attitude or diet.)
I would suggest a 4th option for the Mystery, after exemplar, chimera, and anima - mythic. But how would the result be adjudicated, what are the advantages and restrictions? Maybe a second set of qualities, this time legendary rather the merely mundane. Yes, I can already make a sabertooth lion from an exemplar big cat and the Large Teeth quality, but an exemplar should the best idealized version of the original, not a relative dead for millennia. It’s mechanically sound; but as a part of the character’s story, it seems to miss the point.
I don't know how the Mythic Europe mindset handles "extinct". A creature not on Noah's Ark?
In which case it didn't leave descendants, thus can't be an Ancestor Spirit.
Perhaps the Epitomy inner mystery already deals with Extinct forms? Who would be able to tell in ME?
To be Mythic suggests a Might Score, something Great Beasts get after Final Twilight.
PS just realised ME might know extinct. The plant Sylphium (spelling?)
Or European lions