Has anyone posted the Cubone Theory?
Basically the idea is that a mother Kangaskhan dies, leaving its Kangaskhan cub on its own. The Kangaskhan cub uses the skull and a bone from its mother to use as protection and becomes a Cubone. Then through malnutrition or adapting to a different lifestyle, the Cubone evolves into a Marowak.
This could explain why Kangaskhans are endangered, because some of their cubs are being turned into Cubones. I don't believe it says anywhere that the mothers of Cubones are Marowaks, except for in Red/Blue/Yellow when you have to calm the mother Marowak spirit. But, there could also be Cubones that come from Marowaks and Kangaskhans, which could explain why they are so common.