First things first. Take a minute and check out the first two chapters of Robert Cohen's illustrated Seuss biography. Notice an obsession? Yep, Seuss was a huge fan of elephants, birds, and eggs—and bizarre combinations of the three.
And it didn't start with Horton. Using Seuss's body of artwork as evidence, Seussologist Brian Boyd traces the Doc's obsession with combining elephants and birds in strange ways at least as far back as 1925. Way back then, when Seuss was just a lad of twenty-one, he created this awesome serigraph called "Elephant Presenting Flower to A Bird" (source, p. 359). We are talking a super-rare item here. (If you have to ask how much it costs, you probably don't want it badly enough… or you have kids to feed or something.)
Why elephants and birds? It might have something to do with the contrast. Big thing next to little thing = funny, right? Plus, elephants are heavy and grounded while birds are light and free to fly. It's classically Seussian to combine such seemingly opposing elements:
[Seuss] imagined many variations on the elephants' size, weight and shape: elephants in improbable flight (in 1930, with bird wings and bodies; in 1937, with bee wings and bodies) or sitting on elephant eggs that crack under their weight (1934). (Source, p. 354)
The obsession seems to have peaked between 1938 and 1940. In 1938, Seuss wrote elephant/bird story number one: "Matilda, The Elephant With a Mother Complex." It's similar to Horton (as you might have guessed from the name) but with no elephant in a tree, no elephant-bird, and no happy ending. So maybe it's not so similar to Horton, after all.
Shortly after "Matilda" was published, one of Seuss's elephants, drawn on transparency paper, made its way to the top of one of his trees—or so Seuss bragged. And voilà! Horton Hatches an Egg, elephant/bird story number two, was born…