This feels like retcon bullshit. If you check the beginning of chapter 12.1, there is actually one panel remarking they even ran into a dragon, but it was drawn as a bloody dragon, not a thunderbird. This series is based on a novel, so even if the novel author later changed the dragon into a bird creature, why didn't this manga adaptation make it correct? Instead that amazing one panel, one line "We even ran into a dragon" got turned into a whole flashback and a new character now. Why not have the whole thing there, while they were still in the forest, not as a flashback now. And why call the thunderbird a dragon anyway?