At least with humans you need a genetic diversity among the population to prevent too many births that end up in physical and/or mental deformities. Almost all the reports of cryptids I have read seem to indicate that there are not many of them living near each other which, I think, would cause a major problem in whether or not their species could continue and avoid mental/physical deformities. Theoretically, wouldn't almost all cryptids have died out by now due to that problem (low population density=reproductive problems)?