I have a severe pain in my left leg when I walk. Is there a natural therapy for this?

Yes. If what you describe is claudication, then yes. Regular supervised exercise (walking) has been shown to increase the distance claudicants can walk before pain sets in. Otherwise, if you have pain at rest or a wound which will not heal, then treatment for the poor circulation is recommended.
Maybe. There may be, but first you need to find the diagnosis. It could be arterial obstruction or peripheral arterial disease (pad). This needs to be evaluated because pad can cause no blood flow which leads to an amputation for dead feet.