Emerald - It pines for the mother it will never see again. Seeing a likeness of its mother in the full moon, it cries. The stains on the skull it wears are from its tears.

FR - It wears the skull of its dead mother on its head. When it becomes lonesome, it is said to cry loudly.

LG - Because it never removes its skull helmet, no one has ever seen this Pokémon's real face.

D/P - When it thinks of its dead mother, it cries. Its crying makes the skull it wears rattle hollowly.

---

Maybe the "mother" refers to a clan leader, or matriarch? There could be more than one in each Cubone/Marowak tribe and the skulls of these gets passed down generation to generation of the groups. And when it evolves the skull somehow molds into their head? And yet they get simultaneously replaced in the nick of time for the next newborns?

Really, no clue what I'm saying.

But yeah, Cubone's entries are just masses of tragedy.