In this first stanza, this person is talking about how this creature slithers through the grass. He/she thinks of how some people may have seen him and others may have never, but his sudden presence attracts attention and alertness (his notice sudden is).

Again, in the second stanza, this person just talks about how the snake attracts attention and how it moves in the grass. (We know that this “Narrow fellow” is a snake because this poem has been published anonymously under the title “The Snake” in a journal called the Springfield Republican).

Finally the author gives a hint of how she feels about this creature in the third stanza. When Dickenson says, “Yet when a child, and barefoot, I more than once, at morn,” she most likely means that the idea of walking barefoot among a snake seems appalling to most people, making themselves more vulnerable to a this deadly creature.

Then, in the fifth stanza, this person feels that the other animals around her know how she feels. This is thought because when it says, “Several of nature’s people,” Dickenson means animals. “I know, and they know me; I feel for them a transport of cordiality.” In these lines, this person can feel the friendly presence of other animals, while this one snake is disturbing it.

Now, the way I deciphered the part “But never met this fellow attended or alone” is that this person has never seen any other animal hunting the snake down, but it itself is always hunting others down. Then, Dickenson cannot stumble upon a snake without feeling fear (such as tighter breathing) and shivering at the bone (zero at the bone).

A narrow fellow in the grass
Occasionally rides;
You may have met him,–did you not,
His notice sudden is.

The grass divides as with a comb,
A spotted shaft is seen;
And then it closes at your feet
And opens further on.

He likes a boggy acre,
A floor too cool for corn.
Yet when a child, and barefoot,
I more than once, at morn,

Have passed, I thought, a whip-lash
Unbraiding in the sun,–
When, stooping to secure it,
It wrinkled, and was gone.

Several of nature’s people
I know, and they know me;
I feel for them a transport
Of cordiality;

But never met this fellow,
Attended or alone,
Without a tighter breathing,
And zero at the bone.