To be honest, I have very strong doubts about the origin of the True Wyverians. In the game they say that not even they know their origins, some have abilities that refer to their monsters, others do not. However, what I say is that I do not think that the very first True Wyverians appeared out of nowhere. I do not know if you are familiar with Monster Hunter Stories, the spin-off of the Monster Hunter series, there exist the Raiders, humans who coexist with monsters and who fight alongside them. My theory is that the very first True Wyverians were born through experiments with humans and monsters, combining their cells, disfiguring their bodies and thus giving birth to this new race that we know. "Why would they do that?", you are wondering, well... I think that the scientists who created the first True Wyverians lived in a Raider village and that they had ambitions. They wanted to make humans and monsters closer, both mentally and physically.
What do you think? Do you also have doubts about their origin?