Why was Dickinson so deliberately mystifying, disguising the subject of the poem---a snake, by using so lengthy a title?
While the answer provided regarding the titling of Dickinson's poetry is correct, one could look at the poem in a very Romantic fashion (similar to the one from which Dickinson wrote).
Emily Dickinson was, famously, a Romantic poet. Therefore, her poetry exemplified the characteristics true to the Romantic genre.
In "A narrow fellow in the grass," Dickinson exemplifies her feeling and intuition, freedom of imagination, and her love of the integrity of nature. Without being direct, Dickinson provides the reader with a very specific image--one of a snake cutting through the grass.
The grass divides as with a comb,
A spotted shaft is seen;
And then it closes at your feet
And opens further on.
Here, Dickinson reminds readers that nature can simply brush by us at any time--reminding us that we are at its mercy and that we should regard it with awe.