Emily Dickinson usually wrote about life and after-death subjects. But In this poem she wrote about the animal world, for which we can imply that what she is describing is possibly a snake.
.."A Narrow Fellow in the Grass” .
When she writes fellow, she might be talking about a snake, which is thin and long, and when it moves in the grass causes an effect as if it were "combing it"
--"The grass divides as with a comb.."
Also we can imply that she is writing about a person lying on the grass, that is being approached by the animal.
..."A spotted shaft is seen
Shaft: The dictionary defines it as a long, narrow part or section forming the handle of a tool or club, the body of a spear or arrow, or similar.
Again, it fits the description of the animal above mentioned.
