Goodness and loyalty
Horton is an elephant shown to have a massive heart that Mayzie the bird takes advantage of. When she complains of not having a holiday, Horton agreed to stay with her nest and care for them until her return. However, Mayzie enjoys her freedom so much she does not return. Despite this, Horton cannot bring himself to leave behind the nest and even protects them when poachers find him and steal him to the circus. Indeed, he is so inherently good in nature that he cannot bring himself to let them go even after Mayzie eventually returns. Instead, he adopts them into his own family.
Anti-Feminism
The story has many connotations to the discrimination that mothers, especially homemakers, experience to this very day. Mayzie is portrayed as being 'lazy' because she has to sit in her nest all day waiting for her eggs to hatch. An alternative view point could be that Mayzie felt repressed in her freedom. She longed for escape as all she ever did was look after her eggs/children. When she did eventually get a chance at this freedom, she enjoyed it so much that she was reluctant to go back to her being stuck in her nest.
