The Cave Monster is a creature that is reportedly found inside the Fort Carson cave in Fort Carson, San Andreas. It is said the creature is an escaped test project from Area 69, similar to the Mutants. It was discovered that the myth came from a CLEO mod, which is why it has an odd shape and a texture that cannot be found in vanilla. Their physical appearance is very similar to the Mutants.
The Cave Monster is a false myth, as it is just the product of a CLEO mod.