Like the snake, the speaker in this poem is a key element. Of course, that's not saying much, since really those two are the only identifiable figures in the whole thing. Still, the speaker goes a long way toward accomplishing in the reader the same destabilizing sense of unease that the speaker himself admits to in the poem's last stanza.
Notice that? We said himself. That's because, smack dab in the middle of this poem, we learn that the speaker is… not a woman (as a reader might assume in reading a poem written by Emily Dickinson), but a man. More importantly, he's a man looking back at his youth as a boy. So, we're really at two removes from Dickinson the poet.
A couple of things about this: This is why it's always, always a good idea to talk about the "speaker," as opposed to the "poet," when discussing poetry (they're not always two peas in the same pod, you know).
Why do you think Dickinson does this? Couldn't a girl go walking through the grass and find a snake just as easily? It seems that there's something else going on, some purposeful attempt to pull the rug out from under the reader.
In a poem that deals with the confusion between snake and "Fellow," snake and whip, and nature and people, one way to see the speaker is just as another device to remind the reader that, what is assumed to be one thing to begin with, may not always be what we assume it to be in the end.