At the length of 9 minutes and 48 seconds, "Horton Hatches the Egg" is the longest Looney Tunes or Merrie Melodies short ever made.
The song Horton is singing , but doesn't know the words to is 'The Hut-Sut Song', a popular novelty song from the early '40s.
Peter Lorre was a favorite characterization for the famed Warner Bros. cartoonists, as he tangled several times with Bugs Bunny and Daffy Duck. He was also portrayed as a fish in a Dr. Seuss/Warner Bros.' short, "Horton Hatches the Egg".
In early drafts of the original story, Dr. Seuss could not decide on the name for the elephant. He variously named the elephant "Osmer", "Bosco", and "Humphrey". He finally decided on "Horton", naming the character after his old friend Horton Conrad.