The smallest cat inside the hat in “The Cat in the Hat Comes Back” is Little Cat Z.
Published in 1958, “The Cat in the Hat Comes Back” introduces a sequence of progressively smaller cats hidden beneath one another’s hats. Little Cat A appears first, followed by Little Cats B through V, then W, X, Y and finally Z.
Little Cat Z is the smallest of them all, described in the story as microscopic. Despite being tiny, Z performs the decisive action: he lifts his hat and releases Voom, a powerful force that removes the pink stain covering the snow and restores order.
The escalating alphabet sequence is an important part of the book’s visual and comic structure. A common mistake is to choose Little Cat Y because Y appears immediately before Z, but Z is the final and smallest cat. The character also fits a recurring Dr. Seuss pattern in which a physically small figure accomplishes something unexpectedly important.