I spent a long time staring at this and even went as far as to look it up online to confirm my thoughts (to which I found nothing). So keep in mind this is a matter of opinion as are most thoughts on what things may represent.
When I read this, I believe that the fox is representing life and death (hence twice we've seen the fox). The poem is using the fox to represent life by saying it is a flash of red, leaping through the weeds and then it just ends. All thoughts cease once the fox is gone. The fox is fearing because people are afraid of death.
