Gonna have to echo the "you won't get an exact answer" thing.
Just to give a simple example, Dragons:
A large number of civilizations across the world have Dragons in their Mythology, Lore, Folktales, etc. (Egypt is one of the few that DOESN'T, go figure) There's no real origin of it, you look at sources for Dragons based on multiple different areas, be it actual Dragons, or things that strongly resemble Dragons (I think Mezo Americans didn't actually say they were Dragons, but they had depictions of monsters, dieties, etc. that were very similar to Dragons, so its close enough), you'll get various different answers.
Heck, just Japan vs. China have different interpretations. While they look the same, I seem to recall Japanese Dragons have 3 toes, while Chinese Dragons have from 3 to 5 depending on the status of the person wearing the robe (I think only the Emperor was allowed to wear robes with 5-toed Dragons), and both have their own explanation for this (This is from recollection, so might be futzing details):
The Japanese claim that a Dragon is born with 3 toes, and then migrates west towards China once it matures, growing the other 2 during its journey.
China says the opposite; the Dragon is born with 5 toes, and during its journey east towards Japan, it loses 2 toes.
Likewise, the nature of them as well, like European Dragons have wings and breathe fire, while Asian Dragons lack wings (though still fly through magic and what not), and breathe some sort of "Mist of Life."
Yes, this is a very broad example, as Dragons are some of the most universal fictional creatures across cultures, but there is no single origin for them, and each culture has their own interpretation. This can be applied to various other mythological creatures, naturally.