He waddled down to the creek each day, Chuckling over some secret joke, A proud old man who would not give way To any of the forest folk.
His armor of quills rattled and clicked, Warning all to clear his path. Even the bears knew they were licked; They turned aside in helpless wrath.
Quietly, slowly, he would drink his fill, Keeping a wary eye on me. Then, wheezing, he would climb the hill, Ludicrous in his great dignity.
Every day, a porcupine waddles down to a creek for a drink, and the speaker watches him do it. What makes the scene fun is how the animal is described: he's a "proud old man," chuckling at a private joke, refusing to step aside for anyone. His quills rattle and click like a warning, and even the bears, who could surely overpower him, give up and turn away in "helpless wrath."
The speaker seems amused and a little admiring. The porcupine is slow, wheezing, and faintly ridiculous, yet he carries himself with total confidence. That's the joke of the last line, "Ludicrous in his great dignity." The speaker laughs at him but also respects him, since his dignity is the thing that wins every standoff. The mood is gentle and humorous, with a warm sense of quiet observation. Notice that the speaker also gets that "wary eye," so the porcupine keeps even a human watcher at a distance.