At the end of Chapter 16: Jailbreak, you duel three bad guys named Sergio, Ennio, and Eli. Their names are obvious references to Sergio Leone, Ennio Morricone, and Eli Wallach, who directed, scored and starred in a series of spaghetti westerns with Clint Eastwood.
The only Red Dead title not scripted by Dan Houser, the lead scripter of Rockstar's most popular titles.
Red Harlow's voice is based on actor Clint Eastwood. Eastwood famously played The Man with No Name in Sergio Leone's Dollars Trilogy: A Fistful of Dollars (1964), For A Few Dollars More (1965) and The Good, the Bad & the Ugly (1966).
The game takes place during the peak of the American Frontier, in the 1880s. The next installments Red Dead Redemption (2010) and Red Dead Redemption II (2018) take place after the Frontier days, in 1911 and 1899, respectively.
Red's last name "Harlow" is used by a character in the TV series "The Rifleman" and curiously both Red Dead Revolver and The Rifleman are from the western genre.