Yin Beasts were few in number. However, some Yin Beasts were recorded in the Daxia Empire's history for the sole reason that they hadn't been killed despite multiple appearances.
This meant that the Yin Beast was powerful enough to survive even after a team of soldiers had pursued it on countless occasions. That alone was a testament to the Yin Beast's strength.
So, for such Yin Beasts, a name was given by the Empire in reverence to their might and ability to survive. There weren't many Yin Beasts that reached such a state. After all, before they gained such a status, they had to achieve a certain feat.
Killing an Imperial Meister.
An Imperial Arms possessed enough power to almost destroy a Nightmare. No Yin Beast was capable of surviving that. However, if one survived such an encounter and even managed to kill the Imperial Meister, it meant it was in a class of its own.
Such Yin Beasts were given names, dictating their fearsome might.