Way back in the days of GTA San Andreas, there were a number of rumored easter eggs and mysteries loaded into the game which had some kind of creepy vibe to them. The original bigfoot legend originated in SA, as did a number of ghost stories and other alleged cryptid sightings.
Some of the wilder stories went meta, with tales of tragic real-life events being represented in the game or caused by it due to curses or demons and somesuch. Most of it was your run-of-the-mill horror B-movie silliness, but some of the fans were pretty enthusiastic about these "mysteries" and put quite a bit of effort into creating them.
In the end, many of these were proven false or achieved with mods. Some rumors were debunked by Rockstar themselves, while in other cases the authors themselves came out to say that they were just joking. Either way, very few verified cryptid easter eggs remain today in San Andreas.
However Rockstar clearly took note of the large sub-culture that developed around these spooky easter eggs, and decided to fill GTA V to the brim with them. The whole Mt. Chiliad mystery has countless players poring over a mural found in the game, crafting all kinds of insane and contrived theories as to how to progress.
The actual presence of a Bigfoot peyote in GTA V, which itself is only a step towards a larger mystery, shows that Rockstar really took this angle to heart. However, it somehow hasn't gained the same kind of traction as creepy easter eggs did back in the days of San Andreas.
While mods are a significant part of GTA V, they too lag a tad behind the cultural significance of San Andreas modding. An extremely famous mod called Misterix was once made for SA which collected a number of the most popular in-game ghost stories, often based on pre-existing creepypasta, and made them "real".
Misterix took a handful of creepy creatures and apparitions, and tossed them into GTA SA as hostile NPCs with unique behaviors programmed for them. The mod quickly became a cult classic, achieving great popularity with the monster-hunter (as in the activity, not the JRPG game franchise) crowd.
