In keeping with the tradition of unusually horrific Pokedex entries, I present to you Yamask, a ghost-type Pokemon and number 562 in the Pokedex. Yamask is a shadow spirit looking critter that carries around a mask of a human face, and its red eyes have the smaller red circle below which give it the impression of crying. Its secondary type is the "Spirit Pokemon," and at level 34 it evolves into Cofagrigus.
The reason Yamask carries a mask around is because the mask is actually its human face from its past life when it was a human being. Yamasks were apparently created from human spirits of ancient fallen civilizations, and they retain all of their human memories. In the cartoon, it is shown that if someone puts on Yamask's mask, then they are possessed by the spirit of the Yamask.
The Black and White Pokedex entries are:
"These Pokémon arose from the spirits of people interred in graves in past ages. Each retains memories of its former life."