It’s very weird to call a snake a "Fellow." This makes sure that we know it’s a boy snake, but it also implies that the speaker and the snake "know" each other on a personal level. This familiarity sets us up later for the chilling revelation that undercuts this chummy business in the final stanza.
Occasionally rides— (2)
How does a snake "ride" anything? Snakes don’t have legs. Does this mean that the grass is moving? Already the speaker is describing the snake in amazing ways.
You may have met him? Did you not (3)
It makes total sense that someone would publish this line with a question mark at the end, but Dickinson was insistent that she had it in just the right place. In this version, it's as though the speaker is convincing the reader that, yes, this snake and its amazing properties are known to others, too. So, it's not just the speaker who's amazed.
His notice instant is— (4)
Notice how stating the surprise in a “normal” way doesn’t compare to the moments of surprise that contain poetic imagery. The question is, who notices who?
Have passed I thought a Whip Lash (13)
“[T]hought” sets you up for the amazing transformation of the snake. The snake first exists in the speaker's mind (mistakenly) as a whip, but then "transforms" into the reality of a snake. The reflection seems tinged with amazement in this moment, as the previously-assumed reality in the mind twists into a new, snake-y reality for the speaker.
Several of Nature’s People (17)
Where is the line between expected and unexpected, between ho-hum and amazed? The speaker seems to dance all over it in this poem, just as he dances along the distinction between humans and animals.
And a Zero at the Bone (24)
The sum total of this amazing experience is a deeply unsettling feeling. This description of a physical sensation is amazing in itself, but it reminds the reader that amazement—which naturally accompanies any break with expected reality—is always tinged with fear and anxiety about the encroachment of the unknown element.