I'm not aware of any official horror story involving Pikachu in the Pokémon franchise. However, there have been various fan-made horror stories and creepypastas involving Pikachu, where it is depicted as a malevolent creature with supernatural powers and a thirst for revenge.

In some of these stories, Pikachu is said to have been mistreated or abused by its trainer, causing it to become vengeful and seek revenge on those who wronged it. In other stories, Pikachu is portrayed as a demonic creature that can control electricity and possesses dark powers.

It's important to remember that these stories are not part of the official Pokémon canon and are not endorsed by the creators of the franchise. Pikachu is one of the most beloved and iconic characters in the Pokémon franchise, known for its friendly and loyal personality. While these stories may be entertaining to read, they should be taken as works of fiction and not representative of Pikachu's true nature.