TL;DR: A Narrow Fellow in the Grass

Poem 1096 by Emily Dickinson first published posthumously in 1890 among other poems.

A narrow Fellow in the Grass
Occasionally rides –
You may have met him? Did you not
His notice instant is –
This creature rides in the grass. It doesn’t creep, it doesn’t jump, it doesn’t crawl. It rides. So smoothly he rides that you may not notice him at all and only in an instant may you see him.
The Grass divides as with a Comb,
A spotted Shaft is seen,
And then it closes at your Feet
And opens further on –
As he rides he divides the grass. He doesn’t tramp on it, he doesn’t disturb or wreck the grass, he divides it leaving no perceptible track. It is a spotted shaft. He is long and thin, spotted. And as soon as he goes through the grass at your feet it returns to its normal place to be divided at the next place the narrow fellow rides.
He likes a Boggy Acre –
A Floor too cool for Corn –
But when a Boy and Barefoot
I more than once at Noon
The narrow fellow lives in the swamps where there is not farming, where boys go to play in the heat of the day where it is cool.
Have passed I thought a Whip Lash
Unbraiding in the Sun
When stooping to secure it
It wrinkled And was gone –
The narrow fellow passes as a whiplash, long and thin. Our narrator thinking it might be her whip stoops to pick it up and when she tries to pick up her whip it wriggles away showing it is alive.
Several of Nature’s People
I know, and they know me
I feel for them a transport
Of Cordiality
Those who see our narrow fellow give him respect though he is a narrow fellow. They know him, and he knows everyone. Our narrow fellow is a common sight in this place.
But never met this Fellow
Attended or alone
Without a tighter Breathing
And Zero at the Bone.
And when you see the narrow fellow you need to be on guard. He is dangerous this narrow fellow. With someone or alone when you see him, he will make you fear for your life.

If you haven’t already guessed it…it’s a snake in the grass.