Grizzle is the Robinson's donkey. The family is threatened at one point by the appearance of a giant boa constrictor. The snake is such a menace that the family is afraid to leave their home. They also keep their farm animals, such as Grizzle, in the cave with them until they run out of food for them. At that point, Mr. Robinson decides to have Fritz, on horseback, lead the animals past the boa constrictor and into freedom, so they can graze and be well.

Grizzle, however, foolishly runs away. He runs straight into the marsh, where the boa constrictor lives. Grizzle encounters this snake:

With a cold shudder of horror, we beheld the snake rear itself from its lair, the fiery eyes glanced around, the dark deadly jaws opened widely, the forked tongue darted greedily forth—poor Grizzle's fate was sealed.

The family watches in horror as the giant boa constrictor coils itself around the donkey, then slowly swallows it. Finally, the snake is done swallowing. It lays down, immobilized and heavy laden to digest its meal. Mr. Robinson realizes this is exactly the moment they have been waiting for. They can shoot the boa in the head while it can't move. They do so and are happy to have been freed from this dangerous predator. Ernest writes a poem to commemorate both Grizzle and the occasion, noting that Grizzle died because he was disobedient but also ended up saving the family.