Thousands of years ago, the gods declared man and beast to be equal. But a war raged between the two different creatures with man coming out on top. The king of all gods declared - in hopes of ending the feuding - that man was now the master and beast should serve him.
Nowadays, every human is born with an animal as their familiar: two different beings, but they share one soul. Most are fine with this way of life. But some are not. Some are ready to reverse the gods decision.