Anzu

“A man might befriend a wolf, even break a wolf, but no man could truly tame a wolf.”

— George R.R. Martin, A Dance with Dragons

Anzu are beastlike Spirits of Life and Nature, of wildness and the cycle of life. They are also sometimes called "spirit beasts" or "nature elementals" though they aren't actually elementals.
Anzu are not generally very intelligent but that does not mean they are not dangerous.

Creation: Anzu are born ususally from the souls of animals or particularly wild sentient mortals. They move into the Spirit World just as the souls of mortals do, and there they will find their way into the spirit wilds. After staying there for a signifficant time, they will become anzu.

The foodchain endures: Anzu are temporary things. Animals don't have the same eternal souls as sentient mortals do, and as such they slowly fade away and become part of the spirit world itself after enough time has passed. Until this time the anzu live in the spirit wilds much like they lived in the mortal world, following the laws of predator and prey; eat or be eaten. By devouring one-another the anzu are able to increase their own power, and through that stave off being absorbed by the spirit world for a while longer. With the ever-present influx of new souls from the mortal world, there is no chance of the Anzu ever running out.
Additionally, if an Anzu ever devours a lost soul they themselves become a Shade and gain increased intelligence and no longer need to devour other anzu to stave off the drain of the spirit world.

Mortals: Anzu are favored summons of Druids and Dryads which find it easy to call upon and control these spirits of life and nature. Apart from them, anzu are only rarely called upon.
Anzu like being summoned, as it frees them from the need to hunt and devour others of their kind for a time and thus risking themselves. A permanent familliar bond or being bound as a guardian spirit of a druidic grove will also allow them the same benefit so they will gladly accept such.

Primal Anzu

Primal anzu are exceptionally ancient beings that have devoured a truly tremendous number of souls to stay alive, climbed their way to the top of the food-chain of the spirit wilds and become extremely powerful. These are beings of raw, primal power, ancient spirit beasts of legendary status.
They are sometimes called upon by druids in battle and they wreak havoc amongst the ranks of those they are sent against

An anzu taking the form of a wolf.

Appearance:

Anzu are the spirits of animals. If summoned to the mortal world they will thus take the forms of animals suitable to the environment or in accordance with the will of their summoner. Usually however their spiritual nature is obvious even when they take the form of an animal native to the area.