Simply incorporating the Pokedex entries for some of the Pokemon into the anime would make it pretty fucking dark
.
Lampert: The spirits it absorbs fuel its baleful fire. It hangs around hospitals waiting for people to pass on.
Yamask: Each of them carries a mask that used to be its face when it was human. Sometimes they look at it and cry.
Gorebyss: When it spots prey, this Pokémon inserts its thin mouth into the prey's body and drains the prey of its body fluids.
Drifloon: It is whispered that any child who mistakes Drifloon for a balloon and holds on to it could wind up missing. / These Pokémon are called the "Signpost for Wandering Spirits." Children holding them sometimes vanish.
Cacturne: If a traveler is going through a desert in the thick of night, Cacturne will follow in a ragtag group. The Pokémon are biding their time, waiting for the traveler to tire and become incapable of moving.
Haunter: Its tongue is made of gas. If licked, its victim starts shaking constantly until death eventually comes. / It hides in the dark, planning to take the life of the next living thing that wanders close by.
Banette: An abandoned plush doll became this Pokémon. They are said to live in garbage dumps and wander about in search of the children that threw them away.
Honedge: If anyone dares to grab its hilt, it wraps a blue cloth around that person's arm and drains that person's life energy completely.
Phantump: According to old tales, these Pokémon are stumps possessed by the spirits of children who died while lost in the forest.
Gourgeist: It enwraps its prey in its hair like arms. It sings joyfully as it observes the suffering of its prey.
Duskull: Once this Pokémon chooses a target, it will doggedly pursue the intended victim until the break of dawn. If it finds bad children who won’t listen to their parents, it will spirit them away–or so it’s said. It loves the crying of children.
Glalie: Glalie has the ability to freely control ice. For example, it can instantly freeze its foe solid. After immobilizing its foe in ice, this Pokémon enjoys eating it in leisurely fashion.
Cofagrigus: It has been said that they swallow those who get too close and turn them into mummies. / Grave robbers who mistake them for real coffins and get too close end up trapped inside their bodies.
Mismagius: Its cries sound like incantations. Those hearing it are tormented by headaches and hallucinations.
And those are things that just casually lounge around in tall grass (forget to mow your lawn one day and you're proper fucked). Actual legendary Pokemon are literal gods that can create or destroy the world on a whim.
The world of the "mature" version would be littered with corpses of luckless passerby that didn't reach for their Pokeball in time.
