Gastly

“Almost invisible, this gaseous POKéMON cloaks the target and puts it to sleep without notice.”

— Pokémon Red

“Said to appear in decrepit, deserted buildings. It has no real shape as it appears to be made of a gas.”

— Pokémon Yellow

“With its gas-like body, it can sneak into any place it desires. However, it can be blown away by wind.”

— Pokémon Gold