The first Nasod hybrid wasn't Raven. In fact; there were many before Raven and Kigahen; but none were successful experiments, hence the facility they were constructed at was shut down.

Although, some believe that they were originally in a sleep-like state, making them seemingly unsuccessful, but when the facility closed down--just at that precise moment one had stirred. And that one was the Nasod who had saved a Kigahen as a child.