This video may seem long, however it answers/ shows many different myths concerning GTA San Andreas. All of the monsters you see in this video are MODs. Unfortanately, they cannot be seen without mods. However, should you prove us wrong, then leave a comment below.
Edit:- most (if not all) gta myths are speculation or mods, exluding rat man on gta4... i have seen (something) (maby not rat man but...) it on multiplayer TD match.