8. Shedinja
What is it: Shedinja is the dead shedded skin left behind when a Pokémon called Nincada evolves into its next form Ninjask. Oddly, it will just appear in a Pokéball (except in Generation III, when it copies Nincada's ball) as long as the trainer has at least one in their bag, and a space in their party. This shell that is left behind only has 1 HP, which you would think would make it easy to defeat, but its ability gives it immunity to 12 of the 17 types (unsure if it can be damaged by Fairy moves), making it hard to finish off.
Why is it creepy: The Pokédex entry outlines this "Peering into the crack in its back is said to steal one's spirit." However, this is lessened down to just a mere rumour as the trainers see the backs of their Pokémon during battle, and all seem to be okay. Still it is dead bug skin oddly bought to life.