A narrow Fellow in the Grass Snake

Like a green anaconda found deep in the Amazon jungle, the snake in this poem is a biggie. In fact, he is the biggie. The way you look at the snake really colors your whole approach to the poem. For that reason, we can say that this "Fellow" is an important symbol in the poem, but we can't say that there is one clear interpretation as to what, exactly he symbolizes. Instead, we'll present you with a variety of possible readings. Feel free to pick and choose as you see fit! Ready? Okay…

Cue up the song “Sympathy for the Devil” by the Rolling Stones. Our legless friend has a long history and a bad rap when it comes to literature. The poor guy gets associated with Satan one time in the Book of Genesis and he is seen as an allusion to Satan’s corruption of Eve in the Garden of Eden for the rest of time.

Coming from a religious household, Dickinson would certainly be aware of the biblical symbolism of the snake, but that's not all this "Fellow" can represent here. There are those that argue that the animal (with its resemblance to male body parts) might also be a stand in for sex and sexuality.

Finally, there are those who say, "You know, sometimes a snake is just a snake." By that token, we can look at this guy as one of "Nature's People," a representative of the natural world, yet repeatedly endowed in the poem with human qualities.
Frankly, a case can be made that Dickinson riffing on all of these associations. How so? Well, read on.

• Line 1: We’re never told that the narrow fellow is a snake, but it’s pretty clear that it is. The snake could be a symbol for Satan convincing Eve into eating from the Tree of Knowledge, allowing for the creation of original human sin. 
• Line 3: This rhetorical question doesn’t expect an answer, because the speaker already knows that the listener/reader has met the serpent, in one form or another. Notice how the snake is gendered as a male.
• Line 4: The snake pops out of nowhere, catching the speaker (and us) off guard with a comb simile.
• Lines 13-14: The snake is mistaken for a living whip. The speaker is at first really thrown by the animal. First he thinks it's one thing, and then another. In this way, the snake acts as a kind of metaphor for the general uncertainty that confronts the speaker (and probably us, too) in this poem.
• Line 17: The snake is counted as one of "Nature's People," which is a kind of personification. After all, snakes aren't people, are they? The speaker, though, seems to be making a case otherwise.
• Lines 21-24: Coming across the snake is a deeply disturbing experience for our speaker. Just why that is, exactly, remains open to interpretation. What dangers might snake symbolize here? Sex? Sin? A blurring of the natural and man-made world? An instability in the very fabric of reality? We would happily sit and listen to a case for all of these possibilities, and more!