Remnant Beasts, as a type of monster that existed well before the material realm's human civilization, no one can say for sure when they were born.
The most widely known theory is that all Remnant Beasts' lives and powers actually come from the same source, a powerful entity that left behind neither name nor image. However, in the existent research on the magic side, there isn't any evidence to support this theory.
But belief doesn't require evidence, and there have never been a shortage of people who believe this claim. The "Master" mentioned by Black Ash Dawn actually refers to this unknown entity.
They call it the "Beast Lord."
"The great beast calamity nineteen years ago, while bringing a disaster to the material realm, left behind a wealth of valuable treasures for the study of Remnant Beasts' power."