|“||Charlotte: And that's Ubu, the guy who sits!
Herb: He is a good dog.
In real life
In real life, Ubu Roi (commonly known as Ubu) was a black Labrador Retriever and the mascot of Ubu Productions, a production company that produced sitcoms such as Family Ties. After the credits of every episode, Ubu Productions' closing-tag would famously appear: a still image of Ubu, while a man's voice says "Sit, Ubu, sit! ...Good dog" (followed by the sound of a dog barking).
In the series
In BoJack Horseman, Ubu is presumably still the mascot of Ubu Productions, with a closing-tag featuring Ubu presumably being shown at the end of shows produced by the company.
In a flashback in The Telescope, the famous Ubu is seen at an exclusive party in the 1980s, referenced by Charlotte Carson as "the guy who sits." Ubu sits down in his chair, and Herb Kazzaz says that "he is a good dog" (as an off-screen bark is heard).
He is mentioned briefly by BoJack in Prickly-Muffin, as BoJack imagines the Ubu Productions closing-tag playing after the hypothetical credits of his special moment with Sarah Lynn at the park. BoJack envisions credits rolling past the screen (like at the end of a sitcom episode), followed by the famous closing-tag of "Sit, Ubu, sit! Good dog. *arf*."
- Just like Mr. Peanutbutter, the animated Ubu is presumably a Labrador breed of dog.