Random Trivia For This Title:
- The Mayor receives a goldfish in a fishbowl from a Who who looks remarkably like the Cat In The Hat (he is only seen twice, briefly - once from behind, and later from a distance - and his hat has blue stripes instead of red). Later, just before the first attack by Vlad, the Mayor is resuscitating the fish. When the fish lands back in his bowl, he leans on the side of the bowl just like the goldfish in The Cat In The Hat.
- The first portrait that the Mayor shows to Jo-Jo is, in fact, a Who-style caricature of Theodore Geisel (Dr. Seuss).
- Amy Poehler's character is named Sally O'Malley, which is also the name of a recurring character on [Saturday Night Live] played by Molly Shannon. Both Poehler and Shannon got their careers started on [Saturday Night Live].
- The observatory building is shaped like the Cat's hat from the book The Cat in the Hat and the mayor's skis have the same design as the Cat's in the book The Cat in the Hat Comes Back.
- When Horton is turning his ears into different hats, one of the hats is one worn by Bartholomew Cubbins in the Dr. Seuss book 500 Hats of Bartholomew Cubbins.
- Rudy, Kangaroo's son, mentions in one scene an imaginary friend of his named "Thidwick". Another Dr. Seuss story is Thidwick the Big-Hearted Moose.
- Some of the trees seen in the movie resemble Trufula Trees from another Dr. Seuss story, The Lorax.
- The Horton quote "I meant what I said, and I said what I meant: An elephant's faithful one hundred percent," is actually from Horton Hatches the Egg, another Dr. Seuss story featuring the enduringly faithful elephant. Morton the mouse, who isn't in the original book, is actually named after the baby elephant/bird creature, Morton, from Horton Hatches the Egg.
- Jim Carrey's second movie based on a Dr. Seuss story. The first was How the Grinch Stole Christmas. Both stories have Whos in them. In fact, after the snow falls in Whoville the children can be seen building a 'Grinch' snowman.
- Green eggs (from Green Eggs and Ham) are briefly seen during The Mayor and his 97 kids' breakfast. A Who can also be seen holding a plate of 'Green Eggs and Ham' during the crowd scene at the end of the film.