Friendship
This is perhaps the most important message of the story. Thidwick is kind to all the creatures even when they are not so kind to him. Bingle Bug pretends to be his friend in order to take advantage of his kind nature, as do all the other animals. Indeed, Thidwick eventually realizes who his true friends are and shakes off his fake friends from his antlers. Seuss is conveying the need to find real friendships in one's life and to not accept mistreatment from supposed friends.
Kindness
Thidwick epitomizes what it means to be a kind person. His big heart disallows him to refuse to give free rides despite his inherent displeasure. Perhaps this is a reference to how few kind people exist in the world. Everyone always has a motive in the human world, and this may be what Seuss was getting at through the role of Bingle Bug and the other animals. Thidwick, however, is an inherently good and kind person and represents the traits that people should value.
Karma
The theme of karma is shown when Thidwick sheds his antlers. and by doing so, sheds the animals off him. Seuss is showing to the reader that those that mistreat others and take advantage of them will get their comeuppance one day. Indeed, when Bingle Bug and the others complain that they cannot be left alone as the antlers are their home, Thidwick calls them 'pests' and highlights the fact that their selfish actions led to this.
