Milo,
After decades of studying shamanism (in a scholarly way) I am still not sure what a nahual (or nagual) is. if you accept the principle that our bodies contain a spirit and a soul I might can try to tell you. The nahual could be the soul (or astral body) transformed into any odd creature or a likeness of ourselves OR it is spirit (which is only a light) and how we preceive that spirit based on our power. But i could be wrong. I've heard and read that any attempt to describe it would be futile and inaccurate. Sorcerers themselves seem to be hard put to decribe it, though they accept that "nahual happens."