One Pokémon theory asserts that Kangaskhan and Cubone are part of the same evolution chain, or at least that was the original plan by the game’s developers. As the theory goes, a baby Kangaskhan will evolve into an adult Kangaskhan if it’s mother is still alive when it evolves. However, if a Kangaskhan dies and leaves behind a baby, the Kangaskhan cub will protect itself using the skull, and a bone, from its mother and, consequently, evolve into a Cubone. This Cubone will then evolve into a Marowak and, eventually, evolve into a Kangaskhan, thus completing the evolution cycle.
Some evidence for this lies in the Pokédex entries for Cubone since Generation 1. These have continuously highlighted that the skull on its head belonged to its mother:
“Wears the skull of its deceased mother. Its cries echo inside the skull and come out as a sad melody.”
Believers of this theory also highlight how similar Cubone appears to the baby Kangaskhan that rides around in its mother’s pouch. Additionally, Cubone’s signature move ‘Bonemerang’ refers to a boomerang just like ‘Kangaskhan’ refers to a Kangaroo, both native Australian themes.
It is argued that sometime during the pre-production of the Red/Blue games, the developers back-tracked on the decision to relate Cubone to Kangaskhan after the necessary codes had already been put into the game; they didn’t think a child’s game should have an orphaned baby Kangaskhan wearing its dead mother’s skull on its head.
As there was no time to delete the codes, the developers created a new non-evolving Marowak and moved the old one to an empty slot, without a number; and this catchable Pokemon is M’, Missingno’s sister. This theory has been proven correct since a levelled up M’ evolves into a Kangaskhan.