Symbolism, Imagery, Allegory
The narrator and Kay could have made up their minds about a pet ages ago, but they keep getting distracted by the idea of some "other pet." Check it out:
The time may be now
to make up my mind.
But who knows what other
good pets I might find? (20.1-4)
The "other pet" represents the perfect, ideal pet—the one that they must have. Of course, this other pet doesn't exist; it's all in the kids' minds and is just a barrier that keeps them from making a decision. They're terrified that if they settle on an animal, it will be the wrong one, and that this perfect "other pet" will waltz in and make them regret their choice.
To dig a little deeper into this conundrum, swing by the "What Is This Book Really About?" section.