Emily Dickinson writes from the perspective of a male speaker who stumbles across a snake. Remember, the poet is not always the speaker of a poem. In this case, we know the speaker is a male (who is no longer a child):
But when a Boy and Barefoot
I more than once at Noon
Have passed I thought a Whip Lash
Unbraiding in the Sun. (11-14)
The speaker never directly says that he is discussing a snake. Instead, he references a "narrow Fellow in the grass" who moves like a "whip lash" (1,13). Dickinson uses personification when she describes the snake as a "narrow Fellow." In this moment, she describes the snake as a human, labeling him a "fellow."
The start of the poem seems lighthearted. The speaker seems calm as he sees the snake. He asks readers "you may have met him?" which gives the snake human attributes.
It also seems as though the snake is moving away from the reader after the person and the snake see each other:
The Grass divides as with a Comb,
A spotted Shaft is seen,
And then it closes at your Feet
And opens further on. (5-8)
The grass opens "further on," which seems to show that it is moving away from the man.
The end of the poem, however, feels less light-hearted. The closing lines discuss the fear and anxiety that some people feel when meeting a snake:
But never met this Fellow
Attended or alone
Without a tighter Breathing
And Zero at the Bone. (21-24)
This stanza discusses the "tighter Breathing" that so many feel when they see a snake. The initial lighthearted description of briefly seeing a snake and watching it move away shifts to a description of the anxiety experienced when some people encounter a snake.