It's true that there are Celtic myths and I suppose legends which feature bears and other forest creatures, but generally speaking these aren't common knowledge today. They're too old and lost in the mists of time. Of course around the world there are of course exceptions to this – most noticeably Greek and Roman mythology. But it could be argued in the case of those two, the academic interest in those great historical empires is what kept the stories in circulation over the centuries, rather than the stories themselves.
Most "fairytales" are very modern historically, even if some do have roots in mythology of old. Even stories "of" the time the Bow and the Bear is likely to be set, were written or refined much later – such as the modern "popular" mythology surrounding Robin Hood or King Arthur.
The fact that nobody here, including those who have interests in Celtic history/tales or various mythologies, or even those who live in the country the story is located haven't heard of the story, or one similar shows that if is it a traditional Celtic/Scottish myth, it's not a popular one. It's not like Disney/Pixar is turning around and making another "Rapunzel" or "Little Mermaid" where the story is at worst, vaguely known by the bulk of adults before the film is made. I still personally think it will be a modern take specifically created for the film.
Also, if this is from an ancient myth, I doubt it will be a traditional Disney "Princess" movie in most respects. That's one of the advantages or disadvantages of setting it in a specific country and (presumably) at a recognized period too. It has to have some historical accuracy, no matter how vague, to justify its setting – or else, why bother? Why not have it set in a vague never-existent Celtic never land full of Will-O'-the-Wips, Banshees and sea/loch monsters and fairies? Disney has always played it reasonably smart with their Princess movies, with the possible exception of Pinocchio, as they seem to take place in a make believe Europe, with inhabitants and influences from many different countries.
What I'm saying is, Disney/Pixar, be that taking and adapting a Scottish myth or adapting/creating a modern tale, have by naming the country an obligation to be true and faithful to the country.
Oh and Siren, surely Celtic mythology is more likely to have been inspired by Roman (or Norse) mythology rather than Greek? (Although I know Roman mythology shared many Greek mythological aspects).
Also, from my understanding, Wolves and Werewolves are a lot more common than bears (probably Nordic influences). Well, much more common Celtic mythology from say around 1000 AD onwards.
I'm actually facinated by this thread, and I've looked up "Artio" on Wikipedia and I'm a little surprised by the lack of information. I know Wikipedia isn't always the best of sources, but it only lists 1 actual representation (and its in Switzerland!), and no examples of stories or myths containing the goddess - which again seems to support my argument that the vast majority of mythology from that time is either lost or mainly known now by academia rather than the general population.
_________________Most of my Blu-ray collection
some of my UK discs aren't on their database