The seemingly simple first lines of the poem, like the snake they describe, are deceptively slippery. The speaker introduces "A narrow Fellow," and already the reader gets a sense that some strangeness is afoot.
"Fellow" is an everyday, comfortable sort of word—one that you'd use to describe a guy you met in the street. But what does it mean to call a fellow "narrow"? It's an odd word to choose; if you were describing a thin person, you might call them—well—thin! Or you might call them slim, or slender, or skinny; not "narrow," though. The word has uncomfortable connotations of constriction and tightness; it gives the reader a claustrophobic sense of strangeness.
This strangeness compounds when readers learn that this narrow fellow is "in the Grass." This turn of phrase, in conjunction with the "narrow Fellow," might raise the reader's hackles: it seems readers must be dealing with a snake here.
This might be an allusion to the phrase "a snake in the grass," meaning a traitor, which goes back as far as the ancient Roman poet Virgil. The speaker's mention of the grass here thus isn't neutral: not only are readers meeting a snake, they're meeting a concealed, sinister, and likely deceitful snake.
There's a further hint of this danger in the subtle internal sibilance of "Grass," "occasionally," and "rides"—sibilance that will continue through the rest of the stanza. That quiet hiss and buzz—hidden within the words, not boldly out in front—contributes to the feeling of hidden snakiness.
Even more subtly, the form of these lines gives the reader a sense of unease through their grammar:
A narrow Fellow in the Grass
Occasionally rides -
These lines quietly disorient the reader by running subject-object-verb, rather than the more usual subject-verb-object. This is a delicate effect; it's not so unusual to find writers playing with grammar this way in poetry. But it does help to contribute to the overall effect that, with this narrow fellow, readers are on shifty ground.