So, the Romantic poets revived this classical literary term called “the sublime.” The sublime isn’t only a Long Beach pseudo-reggae band from the '90s, in literary terms it's the experience of pleasure and fear in one moment. Dickinson, we know, was no stranger to the Romantics, and “A narrow Fellow in the Grass” is a good example of how fear can create such a rush that it is pleasurable and that pleasure can be so extreme that it causes fear. (Think about the lure of extreme sports, only here it's snake-hunting.) In the end, though, fear seems to win out. While the speaker expresses interest and wonder in the snake, he ends his story on a chilling and unsettling note.
The speaker is to be more afraid of mistaking the snake for a whip, than he is of the snake itself. The snake inspires curiosity, but having unreliable perception is horrifying. Glasses anyone?
The speaker is afraid of the snake, one of "Nature’s People," because Nature’s People understand him better than anyone else, and that cuts through his defenses. Ouch!