All morphs are ~technically naturally occurring whether they were found in the wild or spontaneously appeared in captive populations. Selective combinations of multiple morphs is the "frankenstein" part.
Line breeding does work to enhance the probability of expressing (and degree of expression) of various phenotypes inherited as a polygenic trait related primarily to color and pattern (tiger coastal carpets for example).
There are patternless scrubs, yes. They have not been verifiably proven to be able to pass this trait to offspring in a dominant/incomplete dominant/polygenic/recessive fashion and as such are simply a natural phenotypic variety until that can be proven otherwise. Patternless rock pythons are a proven recessive means of inheritance.