I'm looking specifically for the idea that it is the human condition to be a combination of beast and God, not just a reference to an individual who unlike most people had aspects of both, or a mythical creature with aspects of both. I'm sure I found an answer after much searching last time I looked into this, but now I can't find it again.
Christian theology has the concept of Hypostatic Union, used to explain how Christ can embody humanity and divinity in one being.
The idea of the dual nature of humans predates this doctrine in that figures like Dionysus are both bestial (as horned god) and divine (as the twice-born son of Zeus.) Herakles is another example of a demi-god with a human side (that suffers) and a divine side that is immortal. Here, the hero burns away the moral part to ascend to heaven.
This idea can even be found in early psychology via Freud's now mythical Id vs. Superego.
(There are likely examples from a wide range of mythologies and philosophies because humans are indeed part animal--we rut and defecate and sometimes behave violently--and part divine in that we can "look upward" and grasp that there is something greater than ourselves.)
The oldest extant mythological text treating this subject is likely Gilgamesh. He's ~2/3 divine, but I wouldn't get too caught up in the exact percentage, and instead recognize that he has qualities of both. Enkidu, his mirror, is initially a beast man that is humanized, and dies, forcing Gilgamesh to recognize his own humanity (mortality).
The concept that I can think of as closest to this "half-divine, half-animal" is the Six Realms of Samsara found in Hinduism and Buddhism. There are six realms through which beings reincarnate:
- Deva Realm = gods
- Asura Realm = demigods
- Human Realm
- Animal Realm
- Preta Realm = hungry ghosts
- Naraka realm = hellish
The human realm is situated between the animal and the divine.
An easy way to understand this (in Christian theology, that is), is that God is said to have created mankind. He also created animals. But while man was created, it was created out of a likeness of God, whilst still being not completely divine. Therefore, anything lesser than an angel and God Himself, could be considered "animalistic," or "beastly."