Cubone may look lovable, but he's in need of some serious grief counseling. Distraught over the murder of his mother at the hands of Team Rocket (!), Cubone wears her skull like a helmet, never revealing his true face.
If Cubone's tragic backstory doesn't depress you, then its constant sobbing will. According to the Pokedex, this sad little critter's tears stain the skull and its cries create a "plaintive and mournful sound."
Jeez, Cubone, buck up. At least Nintendo didn't go with their original name for you -- "Orphon."