Symbolism, Imagery, Allegory
The pet shop (which Seuss dubs the Pet Shop… so it's a pet shop called Pet Shop) is never explicitly described in What Pet Should I Get?, but it's clearly the place where the narrator and Kay will find the perfect pet for their family:
Dad said we could have one.
Dad said he would pay.
I went to the Pet Shop.
I went there with Kay. (2.1-4)
In keeping the actual descriptions of the Pet Shop vague, Seuss is drawing out the reader's imagination. The Pet Shop becomes anything we want it to be—the kids can find all sorts of fantastic and imaginary creatures within its walls in the same way the reader can create all sorts of new critters within the pages of the book.
This vagueness doesn't just encourage imaginations to run wild, though; it also invites the reader to put themselves in the narrator and Kay's shoes. This pet shop isn't too specific to relate to; instead it's just specific enough for readers to fill in the rest of the details for themselves, becoming more engaged with their reading experience in the process.