I've wondered that as well, and I've seen where people have debated this before.

To be quite honest, I can't imagine how his form came to be as part of the storyline, but I did have one guess as to why
a character was created this way;

It seems to fall into the idea of Shinto Kami, which I am not going to go into simply because "Kami" has no real translation to it. Some call it spirits, others gods, but it's really just the essence or "-ness" of everything.

Now as for why the character was created I would guess has to do with the reverence for certain animals inmportant in Shinto beliefs, one of which being the fox.

Others include everything from mythical beast like dragons, to bats. But the fox is revered for it's cunning, strength, resilience and mischivious nature.

I am guess that somewhere along those lines are why Kubo created a character in that fashion, yet to be honest, i can't say for the life of me what his creation story would be.

The only thing that makes sense to me is that perhaps one of these revered animals, in this case a fox, was a strong spirit in the real world, and manifested itself in a human like state in Soul Society.

To be honest, that is the best guess I can put forward.