Supposedly, the name of Big Valley was originally going to be "Great Valley", but the name was already taken; the 1988 film The Land Before Time had the Great Valley named as the most prominent location in the aforementioned original film and its subsequent thirteen direct-to-video sequels, the latest of which released in 2016, during Red Dead Redemption 2's development.
The name "Big Valley" is in line with the names of the two other regions of West Elizabeth (Tall Trees and Great Plains) where the names are preceded by an adjective describing the location.
The Big Valley was a Western television series that ran from 1965 to 1969. One of the show's main protagonists, Heath Barkley, was played by Lee Majors, who also played the voice of Mitch Baker in Rockstar Games's Grand Theft Auto: Vice City. The Big Valley television series itself was based on the Hill Ranch in California’s Calaveras County, whose county seat is San Andreas.