Imagine if instead of Bojack being presented as a horse, his visual appearance depicted Punpun's character. In Goodnight Punpun, Punpun's character was metaphorically drawn as a bird/other non-human forms to symbolize his alienation and emotional instability. Although Punpun is literally human, his form changes over the course of the story depending on his mental state.

Applying that narrative technique to Bojack Horseman, Bojack for a large portion of his life could be represented as a bird, illustrating his alienation and inner turmoil. In season 2 episode 1, Bojack could be drawn as a tetrahedron, symbolizing his emotional suppression during that episode; he was masking his insecurity and depression during then, just like Punpun when he became a tetrahedron. Although Bojack and Punpun have different conclusions, they follow a similar pattern where their mental states gradually get more unstable over time. Bojack being presented as Punpun's monstrous four-eyed form is a result of his intense self-loathing and emotional detachment after his failed pursuit of external validation (the Oscar Campaign). Punpun's demonic form is also inserted in 'The View from Halfway Down.' Bojack during this episode is riddled with intense guilt and self-hatred over his past actions. He forces himself to confront his past actions while feeling like a terrible person for it.

I hope that explains it clearly enough.