Instead of describing Bigfoot, the speaker tells us what a poem about Bigfoot would contain, and it turns out to be everything except the creature. It would have nothing to do with "those interlopers," a word that seems to mean the noisy, gawking people who hunt for such things. What it would have is the woods themselves: a cold stream, a shifting rock, bark ticking in the wind, an owl asking its small questions.
Halfway through, the poem shifts into "you," and we are suddenly the one standing frozen in the dark when the crickets go silent. Your pulse is deafening, your knees won't bend, and your feet cling to whatever ground is left. That is the real subject: the feeling of being watched, or of nearly meeting something wild.
The mood moves from quiet and wry to tense, then ends strangely. The poem itself "would cry, possibly for joy" and then "hiss," as if awe and menace were the same moment.