Shedinja

There are old stories in these parts, about a mysterious immortal Pokemon that appears bearing strange gifts. But these gifts cause chaos and it is rumored that this legendary is nothing but a devil...

One day, the mysterious Pokemon appeared in form of a Scyther to a young boy who liked nothing better than to catch bugs. As soon as he got out of school, he would run into the woods and search out newer and bigger bugs. When he saw the Scyther, larger than any other he had seen, with sharper scythes and longer wings, he was overjoyed. The bug catcher had his newly evolved Ninjask weaken the Scyther, and then prepared to capture it.

But then the Scyther dropped to its knees. "Oh please, don't catch me," it said. "There is something I must protect in these woods, and for that I must remain free."

The bug catcher dropped his jaw and the Pokeball. "You... you can talk? I've never heard any other Pokemon talk before."

"I guard a powerful magic, and thus it has transformed me. Please, let me remain."

The boy was disappointed in that he couldn't have the talking Scyther, but nodded. "Okay. I shouldn't take you from your task."

The false Scyther bowed its head. "Thank you; I am most grateful. Kindness like yours should be rewarded. Do you have the shell from when this strong Ninjask was formed?"

Proud that his Pokemon was being acknowledged, he nodded again. "Yeah I do. I think the shells are neat, but my mom thinks they're creepy." He brought out the hollow shell. "I was going to hide it in my secret base."

The Scyther took the dropped Pokeball and the shell. It summoned a shadowy mist to surround both. When the mist departed, the shell was floating on its own. The Scyther handed the ball back. "There. This is a Shedinja. It is stronger than it looks, but you must never look into the crack running down its back."

"Why not?"

"You must never look into its body."

"Well, okay. Thank you, Scyther." The boy headed back for his secret base.

There came a crazed laughter echoing through the woods.

The boy turned back, but saw nothing. The Scyther had vanished. It might have just been the local Pokemon, except that he'd never heard one sound like that before.

The Shedinja never moved. It had wings, but they were stiff. It hovered and followed him, but no part of his body did anything. Not even twitch. If it made any sounds, it was like a faded hiss or crackle. Some people shuddered when they saw the bug catcher's Shedinja, making signs to ward off evil. But he liked it, even if it was ghostly.

It was in Pokemon battles that the Shedinja showed its true power. Most attacks couldn't touch it. Shedinja would endure torrents of water, vine whips, tackles, scratches, body slams, dropped rocks, and displays of sheer cuteness without fail. What attacks did strike it, however, would knock it out instantly, things like fire or bird attacks. The bug catcher was careful not to look into the crack while battling and also to watch out for situations the Shedinja would be frail in. Otherwise, he still liked it.

He did make sure to warn his little brother. "Don't look into the crack on its back or something bad will happen."

"Like what?" the little brother asked.

"Just don't."

The little brother wasn't satisfied with this response. One night, he got mad at his brother and decided to disobey his every word. After the bug catcher had gone to sleep, his little brother snuck in and stole the Shedinja's ball. He released it, then looked into the crack.

None can say what he saw there, for the moment he saw it, he became a zombie.

He walked to his parent's room, carrying the shell. His mother shook her head. "Oh, why do you have your brother's creepy Pokemon? Put it back."

"Look inside," he replied in a flat voice.

Thinking the small boy was just tired, she went ahead and looked inside the hollow. Then she gave it to her husband. "Look inside."

When the bug catcher awoke, his entire village had been turned into zombies. Terrified, he recalled the Shedinja to its ball, then ran for the next village over to speak to the priestess. "My Pokemon is evil and turned everyone into zombies," he said with tears falling down his face.

She patted his back, but then said, "No Pokemon is evil, save one. This one you have simply does not understand what it is doing. I will purify it so that no one else turns into a zombie."

Although the strange Scyther was never seen again, the Shedinjas appear sometimes with trainers of Ninjasks. It is not known if they are all created from the power of the mysterious evil legendary. But most people don't take the chance of peering into the crack running down the back of the Shedinja.

…

Ruby entry: Shedinja's hard body doesn't move - not even a twitch. In fact, its body appears to be merely a hollow shell. It is believed that this Pokemon will steal the spirit of anyone peering into its hollow body from its back.

Kind of based in an American folklore style, in particular the southeastern states. And if you didn't pick up on it, the Scyther was a devil, not a Pokemon.