Werebeasts

Thought to be a curse by those who encounter it is simply a mutation caused by interaction with natural magic sources by wild animals, which allows a victim of an attack to be infected with a magical disease spread through bodily fluids that changes their form when the moon is in its greatest presence.
 * Overview

It is believed that many beastmen originate through two werebeasts mating with one another, creating new species.
 * Speculation