“Back home, legends would be passed down throughout generations. Those legends would always have an inkling of truth, no matter how small. Maybe Equestria has legends of strange, hairless bipeds scattered throughout their history?” It could also explain how Luna might not have known about humans before. She simply isn’t aware of the old legends that contain descriptions of humans, or human-like, figures. “I would need to account for how the descriptions may have changed over years of being retold, but it might just be my best lead so far.