- Actor-Inspired Element: In the original script, Horton confuses Vlad with a good Vlad. It was Jim Carrey's idea to have the good Vlad be a bunny that makes cookies and shows up in the ending.
- Channel Hop: Until Warner Bros. obtained movie rights to several Dr. Seuss books in 2018, this was the only film adaptation of a Dr. Seuss book not to be produced and distributed by Universal, instead being done by 20th Century Fox.note
- Copiously Credited Creator: The 1970 special had Chuck Jones credited as a director, co-producer (with Seuss), voice actor (with June Foray), and storyboarding (with Bob Ogle). Dr. Seuss himself has billing in the title for writing the book, plus co-producing with Jones (credited under his real name, Ted Geisel), lyrics, and teleplay for the adaptation.
- Descended Creator: In the 1970 special, co-director Chuck Jones as Junior Kangaroo, Vlad, JoJo and some additional roles.
- Development Hell: Blue Sky Studios reported to have the rights to Horton Hatches The Egg years ago, but nothing came of it.
- Doing It for the Art: Let's just say Blue Sky knew exactly what they were getting into when they decided to adapt a book by one of the most beloved children's book authors of all time.
- Name's the Same: Morton is the name also give to Horton's bird elephant son from Hatches An Egg in some adaptations.
- The Other Darrin: The 1970 special is very inconsistent in voicing background characters. This is most notable in the "Mrs. Toucanella" number, where the birds seem to have different voices every line.
- Throw It In!: The script called for Ned to simply say "No" when his wife asks if he believes that there's a giant elephant in the sky. Steve Carell ad-libbed the elongated awkward laugh he makes beforehand.
- Unintentional Period Piece: A Who references "WhoSpace", a reference that has basically become nonexistant after the emergence of Twitter and other social media sites.
- What Could Have Been:
- Sour Kangaroo was originally going to receive a fair punishment for her actions, but due to being against Dr. Seuss' wishes of peace and friendship, Kangaroo eventually makes a HeelFace Turn instead.
- It was originally going to be made much more clear from the get-go that the Whoville city council had far more power than the mayor, as they outright threatened to remove him from office should he not keep up a happy face.
- Sour Kangaroo originally had a scene where she had the Wickersham Brothers burn Hortons house to further symbolically kick him out of the jungle, and the end of the film would feature them rebuilding it as penance. It was cut because it was believed to be a step too far by Sour Kangaroo if she was to later have a HeelFace Turn.
- As noted below, the film originally had a Bittersweet Ending with Horton bidding goodbye to Whoville forever as he returned the speck to its rightful place.
- Plans for a sequel based on Horton Hatches the Egg was planned but due to unknown reasons, the idea was scrapped before this film premiered.
- Word of God: In the DVD Commentary, the directors wanted to avoid Horton leaving Whoville behind so they replaced it with the scene where they sing at the end.
Trivia / Horton Hears a Who!