Even though the simplest answers are "trade", "hoax", and "theft" - they sort of miss the bigger picture here. Therianthrope iconography is quite real, and does span across these cultures and time.

I have three guesses for the root cause of this. In reverse order of plausability.

1. Creatures like this really existed and were likely the product of some sort of genetic engineering (the platypus supports this)
2. The iconography is symbolic, not literal - it could be anything from mundane characteristic - swift, graceful, eats by pecking with a beak - to perhaps something more abstract and interesting such as those people depicted had the ability to fly (most likely by mechanical means).
3. Ornate headdresses and costumes like we still see today across many primitive tribes.