There are many figures of speech in this poem. Even the central phrase, a "narrow fellow," can be described in various terms. First, this is an example of circumlocution—Dickinson does not say outright that she is talking about a snake, but rather, uses more words than necessary to hint euphemistically at this fact. Next, it is an example of personification—the snake is not literally a "fellow," nor does he "ride" as a man might ride a horse; Dickinson is ascribing human characteristics to an animal. This personification continues in Dickinson's use of "he" and "him" pronouns to refer to the snake, and in the idea of having "met" him. This is a verb we usually only use about other humans.
There is also a simile in the second stanza: the grass "divides as with a comb." This creates a very evocative mental image of the grass being neatly parted by the snake's passage through it, like a comb parting hair.
A further metaphor, "Nature's people," is used to describe other animals—this, too, is a form of personification. Remember that personification is a type of metaphor, although of course many examples of metaphor are not also examples of personification.