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Kangaskhan being related to the Marowak line is not just a theory
>Cubone and Marowak are the orphan Pokémon
>their bodies resemble Kangaskhan, the mother Pokémon
>they always appear near each other
Its obvious that Cubone is a baby Kangaskhan which lost its mother, I don't know if they had planed for Marowak to evolve in to Kangaskhan, but at least they are related in that way.