The Horse That Couldn't Be Broken was a book about war and justice written by Butterscotch Horseman.
Butterscotch was egotistical, delusional, and stubborn when it came to his writing. He had dreamed of becoming a beatnik and publishing the next “great American novel."
However, Butterscotch had barely started working on it by the time of BoJack's birth, he didn't even have a plot or characters developed. He only a vague idea, that involved the theme of "the great American dream, promised by society," and this would continue for the next thirty-five years.
Butterscotch Horseman did release his novel, titled The Horse That Couldn’t Be Broken, sometime in the early 2000s. Although it's more than implied it wasn't successful, more just him accomplishing a pipe dream—which was all his "talent" and "ideals," around his writing broke down to in the end. It's blatantly obvious, he had no talent and just nonsensical ideas. His spite and narcissism for the world around him, is what fueled his ego, and made him delusional about his faith in them.
- In Free Churro, it is revealed reveals that the book gained almost no attention, and the one review it received "tore [the work] to shreds," which implies that Butterscotch was not the talented writer he believed himself to be.