That Butterscotch committed suicide after seeing his horrible book reviews and Beatrice made up the story of him and the critic.

There’s not a lot of evidence for it but it makes more sense than what we’re told, and from the time they both grew up in suicide was seen as “the cowards way out” since mental health wasn’t taken seriously. Beatrice could’ve lied to protect her reputation among her socialite friends and keep Bojack from thinking his father was “weak” (also deep down she may have wanted to protect him from what that knowledge would do, even if she would never admit it)

I’m pretty sure it’s not true, and it’s more of headcanon really, but I think it’s neat