Red: Because it never removes its skull helmet, no one has ever seen this POKéMON's real face.Yellow: Wears the skull of its deceased mother. Its cries echo inside the skull and come out as a sad melody.Gold: If it is sad or lonely, the skull it wears shakes, and emits a plain tive and mournful sound.
Prof. Lotus' Notes: A popular fan theory is that Cubone are the little Kangaskhan that lost their mother. However, Professor Lotus is here to bust the credibility of that theory. Let's start with the similarities, it is plausible that the baby in Kangaskhan's pouch would grow up to be a cubone, the size makes sense. However, two plot holes there, the 'horns' on Kangaskhan are actually its ears, so the physiology is different. Second, Kangaskhan's Mega Evolution pokes holes at this, as you see the full baby grown, and we've seen Cubone's face partially in the Pokemon Carddass originally labelled 'File No.104'. Finally, and I think this is the crux to this theory, it's the typing. Normal (Kangaskhan) against Cubone's Ground Type.