The Cave Monster is a creature that is reportedly found inside the Fort Carson Cave, near 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 modification, 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 fact that the creature came from a CLEO modification drops a bombshell on the myth's validity.