Not to be confused with Aliens, Mutants are said to be dangerous, vicious beasts that will attack unprovoked. Although they appear to be the same creature as the Fort Carson Cave Monster, they are much harder to kill with average weapon and are usually found at the Verdant Meadows Airport, or around Area 69.
There does not seem to be much proof pointing towards the existence of Mutants. Other than various videos or pictures on the internet, most of the sightings of Mutants in GTA San Andreas are the result of mods.
- Since because the Mutants are found in the desert part of the map, it's possible it can be a reference to the 1977 movie The Hills Have Eyes.