Horton Hatches the Egg was adapted into a ten-minute animated short film by Leon Schlesinger Productions in 1942, released as part of Warner Bros.' Looney Tunes series. The short was directed by Bob Clampett.
In producing the cartoon, Clampett's unit did not use a storyboard as was the custom practice. Instead, they sketched and wrote additional ideas for the cartoon in Clampett's copy of Seuss' book. Several gags not in the original book were added for the cartoon. A fish caricature of Peter Lorre says, upon seeing Horton pass by in a ship, declares, "Well, Now I've seen everything," and promptly shoots himself in the head (this scene is often edited out of modern broadcasts of the cartoon). Mayzie also does a breathy Katharine Hepburn impersonation ("raaaaaaaa-hlly I do") at several points. The cheerful elephant (along with his son, at the end) sing a popular nonsense tune of that era, "The Hut-Sut Song" by Horace Heidt.