So what if Cubone is Kangaskhan’s pre-evolution? But in order to evolve into a Kanghaskan it needs a mother's attention. But say if the mother died, the little Cubone would take it's mother's skull as protection, and a bone, then evolve all by itself into a beautiful Marowak.
You may wonder why it doesn't evolve into a Kangaskhan, well Pokémon games are well known for items having an effect. So it's the skull on Cubone that evolves it into Marowak.
Also what about the stomach pouch on Kangaskhan's? Well a Kangaskhan is a female-only species with no male counterpart, however a baby Kangaskhan's gender is not mentioned anywhere. So Cubone's are male baby Kangaskhan's, which is why they don't have a pouch, although if these were to evolve into an adult Kangaskhan, it would become female.
So, what do you think? It's well thought through and actually makes sense... I'd like to believe this is true, and I would love the creator of Pokémon to confirm this theory!
If you like this theory, check out the Venomoth / Butterfree Theory.