In episode 3 of season two, Henry Winkler has a conversation with a ferret lady at Herb's funeral, politely telling her off for trying to pitch something to him before finishing with 'I'll let you get back to your business', and she walks off to join two other ferrets on screen. A group of ferrets is known as a 'business'.
Many of the pictures in Mr. Peanutbutter's house are of dogs' butts. Dogs sniff each other's butts to greet and get to know one another, and also have poor eyesight compared to their amazing senses of smell, so of course Mr. Peanutbutter would have pictures of dog butts rather than dog faces.
At one point, Mr. Peanutbutter exclaims how New York City makes great salsa. This is a reference to an old series of Pace Picante Sauce commercials from the 1990s, in which cowboys would become disgusted that the Brand X picante sauce they're about it eat comes from New York City instead of San Antonio.
Any viewers familiar with SCUBA diving will understand immediately why everyone is so offended by Bojack repeatedly making the 'thumbs up' gesture in "Fish Out of Water." Among real life divers, the thumbs up means 'go to the surface' - which fish would interpret as 'go kill yourself' since they cannot breathe air.
Many of the stripper/prostitute orcas occasionally showing up have floppy dorsal fins, which one of the in-universe advertisements even highlight as an "appeal" to the whales. In real life, orcas with floppy dorsal fins is a phenomenon generally found with orcas in captivity and is theorized to result from their being unhappy with their living state.
Bojack and the other horse characters in the show are not colored randomly; their colors are based on distinct real life horse colors. Bojack is a bay horse, Beatrice Horseman is a palomino horse, Butterscotch Horseman is a Grulla horse, Secretariat is a Chestnut with chrome, Honey Sugarman is another Chestnut, Joseph Sugarman is a Buckskin, Crackerjack Sugarman is a Liver Chestnut, Hollyhock is a Red/Strawberry Roan. Meanwhile, Dr. Champ the therapy horse appears to be a specific breed of horse as he strongly resembles a Norwegian Fjord horse - production designer Lisa Hanawalt actually has a fjord horse named Juniper.
In "Free Churro," the lizards that make up the entire funeral audience are mourning geckos.
Princess Carolyn is handed a list of clients who were represented by a recently-deceased agent: Sam Neill, Laura Dern, Jeff Goldblum, Samuel L. Jackson, Wayne Knight, and Mr. Peanutbutter, who's the only one among them not crossed-out. That's not just a random list of real-world people—they're the still-living big-name actors from Jurassic Park.
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