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.