cubone is a baby kangaskhan and here’s the story and why it didn't, when they just made pokemon cubone was supposed to evolve into kangaskan but when pokemon red and blue was about to come out, game freak realized that they didn't have enough time to finish marowak so they made a different marowak and moved the old one to an empty spot and the one on the empty spot became missingno but if missingno levels up in any way it will evolve into kangaskhan so cubone is still a baby kangaskhan