- Accidentally Correct Writing: Elephant birds were real, though they looked nothing like the one in the book.
- Creator Backlash: Whether or not he liked the Looney Tunes adaptation is unknown, but Dr. Seuss was severely ticked off for only being paid $50 for the rights!
- What Could Have Been: In Seuss's original draft, Horton and Mayzie's attitudes about the agreement were a complete reversal of how they ended up in the final draft; instead of a reluctant Horton being pressured by a shiftless Mayzie to watch her egg, Horton volunteered for the task and Mayzie was reluctant.
- Plans for a film adaptation to be a sequel to Horton Hears a Who! was made but due to unknown reasons, the idea was scrapped before the previous film premiered.
Trivia / Horton Hatches the Egg