The Horse and Wagon
When Marco sees the horse and wagon, it sparks his creativity, inspiring him to create a good story to tell his father later on. He is disappointed by the mundane reality before his eyes, and instead decides to exaggerate this a little, later concluding that: "I'll say that a ZEBRA was pulling that cart!"
Marco's father
Marco's father frequently shuns Marco for his imaginative nature, telling his son that he should not lie or exaggerate. As such, Seuss presents Marco's father as a symbol for parents who stifle their children's creativity, which should be celebrated rather than censured.
Marco
Marco himself can also be seen as a symbol. His vivid imagination inspires the fantastical events of this story, and we see that as a child he is imbued with a sense of wonder. As such, we might see Marco as symbolizing the vivid imaginations of children.
Imagination
Imagination is a key motif in this story and is seen throughout. This is because the protagonist Marco sees the world through rose-tinted glasses, and has a vivid imagination.
Eyesight
Eyesight and the act of seeing is something that is referred to frequently in this story. For example, Marco's father tells his son to "keep [his] eyelids up," and later tells him that his eyesight is "much too keen," suggesting he uses his imagination too much. Overall, this represents the idea that Marco and his father see the world differently.
