It's not that big of a question. We're just creatures, it'd be as forward as asking what a human is, I guess.

[...........

And yet he seems to have to think how to even explain it! His paw drums against the floor, and he sighs, settling on a simple way of defining this:]

We're just monsters. Humans and Kemonogami used to be one entity. Something happened, no idea what, and they separated and now we live in different worlds. Unfortunately each one of us is tied to a human's soul.

['Unfortunately' because he has some serious grudges to his human partner and his tone turns harshes when he says that last part.

But yeah, guess Kenonogami pop into existence when humans are born, since each one is said to be a small piece of a human's soul!]

Before you ask, time flows different in each world. A human's lifespan is worth several millenia for a Kemonogami.