Okay! Off we go. Let's see…well, your guess is as good as ours as to who this fellow is. We get just a few details about him: he's narrow, he's in the grass, and occasionally he takes rides.
Hmm! Well, since he’s narrow and riding "in the Grass," we'll assume that were talking about a snake. Hey! Look at us. Riddle #1: solved.
But let’s not get too settled on that whole snake thing, because it’s shaping up to be a weird poem.
For example, this snake is called a "Fellow" who "rides." Rides what? Do they make little bikes for snakes? He's being treated more like a human than an animal so far. That, folks, is called personification. We wonder if this treatment will hold up through the rest of the poem…
You may have met him? Did you not His notice instant is -
The speaker is asking us directly if we have met the snake. It's like he (or she) wants to verify their experience by checking with us first.
“Did you not” just seems to be hanging out there after the question mark.
You’d think the question mark would come at the end of the line: “You may have met him. Did you not?” But it turns out that when the poem was first published, Dickinson’s sister-in-law or the publisher took it upon themselves to put the question mark at the end (See “In a Nutshell” for that story and hurry back!).
“His notice instant is” is also vague, mainly because of the twisted syntax. But this isn't just an exercise in Yoda-speak. This line is purposefully worded in such a way that you just can’t be sure if he notices you instantly or if you notice him instantly.
This is a double whammy in a way: both meanings, at the same time, suggest that the snake and the person scare each other in one simultaneous moment.