No Matter How Small
The overarching theme of the story is expressed in its most famous line: “A person’s a person no matter how small.” The infinitesimally small civilization of Whos that live in the speck Horton places into the flower can represent any underprivileged, under-appreciated, underpowered, or misunderstood agency. Horton disregards the mockery of his assurance that these tiny creatures exist despite not being able to see them and does all he can to take care of them. It is a tremendous lesson to take to heart.
Trust and Faith
Horton has faith in that which he cannot see while other animals in the jungle make this very aspect of his personality a thing to be mocked. Horton has faith that what members of this invisible civilization require from him can be trusted. He goes to great lengths to help those he cannot see and whom he is told do not really exist rather than give in to peer pressure to go along with the majority of doubters. Some have read the story as a commentary on faith in God and the acceptance that God does speak to true believers even if He does not appear to them.
Bullying
Horton is a good, decent elephant. He is also accused of talking to things which are not there. For a contemporary audience, there is much about Horton that might be coincident with his being somewhere along the spectrum of autism. This makes the story’s relevance a bit more precise in its relationship to the theme of bullying because Horton today has become a bit more identifiable as a particular type to be victimized. Of course, themes related to bullying have been identified with Horton’s treatment by the monkeys and kangaroos and other animals since publication, but with the identification of autistic characteristics and the ease with which they can be applied to Horton’s fundamental traits, this particular type of bully targeting now seems much more starkly outlined.
Perseverance without Profit
The Who civilization living within the speck make a request of Horton for help and he obliges. But it is not his job. Nothing is forcing Horton to carry out this request to help entities he cannot even see. He recognizes a job that someone should do and enforces upon himself the takes of making sure it is done. In the face of multiple difficulties—any one of which would be more than understandably engaged as an excuse not to do it—Horton makes sure this task is completed and his job is fulfilled. The elephant perseveres and for his effort is rewarded with nothing tangible but a wealthy supply of the intangible.
