19.
I will vote up. Pokemon can be surprisingly poignant sometimes like a Cubone mourning the death of their mother by wearing her skull over it's face. This also raises more questions than it answers, because Cubone always masks it's face with that skull. If taken at face value, then that would mean every cubone that you encounter lost their mother.
There are some species in our animal kingdom that die shortly after mating and giving birth, certain kinds of marine life for instance, but the whole Lavender Town side quest I think undermines the idea that explanation, and the darker one that Cubone kill their mother. The cause of death for Cubone and Marowak was humans poaching them, not something happening in nature.
More noteworthy is the fact that you can breed Cubone in-game, but fortunately the parent doesn't die, yet the newborn hatches from the egg wearing the skull helmet that's supposedly it's mother's remains.
The best I can do to explain it is that maybe there was once an ancient cubone that hid it's face with it's mother's skull in grief after she died, and then the species overtime evolved to look the way it does today because they are all descended from that ancestor.