A narrow Fellow in the Grass (1)
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.