The Pokémon video games, movies, and anime are populated by a surprising number of disturbing creature designs, especially when it comes to Ghost-type Pokémon. Several have Pokédex entries that mention death, kidnapping, and other subject matter most would assume too scary for a children's series, and Yamask, the Spirit Pokémon, is one of the creepiest.
Many dark secrets permeate the Pokémon universe. Creatures with cute exteriors hide unnerving true intentions, and The Pokémon Company often hints at bits of lore that break the Pokémon universe's kid-friendly utopia, like that fact that people clearly eat Pokémon. Unlike many children's properties, though, Pokémon's creepy backstory often goes beyond fan theories, instead directly confirmed by in-game text via the Pokédex.
These Pokédex entries range from mildly spooky do downright violent. Drifloon's, for example, says the Pokémon kidnaps and kills children, dragging them away "to the afterlife." Yamask, a Pokémon introduced in Generation 5's Pokémon Black and White, has a similarly creepy Pokédex entry, which implies it began its life not as a Pokémon at all.
Yamask Origins: A Dead Human That Became A Ghost-Type Pokémon
According to its Pokémon Black Pokédex entry, each Yamask "carries a mask that used to be its face when it was human." Since every Yamask's mask looks identical and only vaguely resembles a human face, it seems it's not a representation of the Pokémon's former face but literally its face turned into a mask. What's most disturbing about Yamask is that it still remembers its life as a human. "Each retains memories of its former life," White's Pokédex notes, and it is known to sometimes look at its mask and cry - presumably because it longs to be a human again.
The idea of children capturing the spirits of dead humans and forcing them to fight other Pokémon is troubling, to say the least, but Yamask isn't actually the only Pokémon said to be a former human. Kadabra's FireRed Pokédex entry says it was "a boy with extrasensory powers" who suddenly awoke as a Pokémon one morning, Moon's Pokédex says Gengar "was once human itself" and Froslass is "soul of a woman" that possessed an icicle, and Sun's Pokédex says Phantump are tree "stumps possessed by the spirits of children who died in the forest."
Notably, the language of these scary Pokédex entries varies from game to game, sometimes declaring the creatures' origins as fact and sometimes qualifying them with phrases like "legends say" or "is said to be." Some fans believe the Pokédex is a sort of unreliable narrator, so it's hard to say if these Pokémon really were human. But Yamask's origins do appear more definite, however, as its human face is clearly incorporated in its conceptual and visual design.