These Pokémon arose from the spirits of people interred in graves in past ages. Each retains memories of its former life.
– Pokemon White Pokedex
Based on its Pokedex entries, apparently Yamask used to be human. Wait, people in Pokemon world die? And their spirits become Ghost Pokemon? The Yamask family is a bit… strange, in terms of appearance and origin. Yamask itself isn’t as bad as its evolution though, in fact, it sort of, kind of, is cute.
It’s the only pure Ghost type Pokemon in Gen V. Stat-wise, it’s a defensive wall, since it is immune to two types, and has high Defense and Sp Defense. Its ability Mummy, makes any Pokemon that makes contact with it also get the ability Mummy (and it passes on, for example, in a double battle, if another Pokemon makes contact with that Pokemon). It essentially gets rid of the foe Pokemon’s ability, which may be good if the foe Pokemon has a good ability (like one that gives it immunity for example).