You cannot judge a person (or a horse or bird, for that matter) on WHAT they are but by WHO they are. A child is not born with prejudices; those are learned ideas.
At 9 years old, the Author chose the name of the book based on a horse and a bird, the definition of the word reflects a larger meaning behind the story of friendship. Merriam Webster Dictionary defines horsefeathers as something not worth considering; interjection; rubbish; nonsense; bunk. It is nonsense, rubbish, and bunk to stop someone from being friends with anyone they choose.
It is Horsefeathers.