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.
That's just my analysis of what is happening.