This Pokémon has electricity-storing pouches on its cheeks. These appear to become electrically charged during the night while Pikachu sleeps. It occasionally discharges electricity when it is dozy after waking up.
Whenever Pikachu comes across something new, it blasts it with a jolt of electricity. If you come across a blackened berry, it's evidence that this Pokémon mistook the intensity of its charge.