Fan theory time! Know that this is a theory and therefore is not outright canon, even if it sounds plausible!!
Cubone was originally supposed to be the the child of Kangaskhan.
The evidence is that the skull of Cubone looks like the head of a Kangaskhan.
Cubone's signature move is called "bonemerang", a mix between bone and boomerang. Boomerangs are a stereotypical Australian thing, and Khangaskans could be based on kangaroos, stereotypical Australian animals.
The final piece of evidence is that there is a glitched pokemon in red and blue (kind of like missingno), if you manage to level this pokemon up there's a chance it can evolve into a Kangaskhan. This could imply that Kangaskhan was supposed to have a pre-evolution