An heir is a mundane living thing that has been given a magical lineage, transforming them into a mythical creature. Humans are a preferable choice of heir, and so there are many heirs on Earth.
All heirs have access to their magical parents' abilities, and many have the power to transform themselves into a fully-fledged mythic form. Others may be permanently transformed in some way, taking on traits or even body parts from their patron mythics. Many heirs find themselves unable to live with their birth families. Some abandon their humanity completely, living among their magical ancestors or traveling to other worlds for their own safety.
Unlike witches, heirs struggle to learn abilities outside of their patron mythic's sphere of influence. They can train their natural strengths, and even expand their lineage with specialized training, but they cannot branch out into multiple fields of magic at once.