The Name of Polanski's Dog Here
At some point in Once Upon a Time in Hollywood, we get a "morning moment" of hot young director Roman Polanski having a cup of morning coffee in his backyard -- a reminder that no matter how rich and famous you are...life is life. You have a cup of coffee in the backyard and start your day.
Polanski's dog runs up to him and Polanski shoos him away saying something like "Go away, Saperstein!"
Its a nice "in joke" from writer-director Tarantino.
Saperstein -- DR. Saperstein -- was the name of the villainous warlock pediatrician played by Ralph Bellamy in Polanski's 1968 hit "Rosemary's Baby."
So..Polanski WOULD go on and name his dog after a character in his biggest hit. (To date: Chinatown was coming...)