Gastly

Gastly (ゴース, Gōsu?, Ghos in original Japanese language versions) Spirit in the Beta versions of the English language versions of Pokémon Red and Blue consists of a dark sphere (its "head") surrounded by a light purple gas. A Gastly has two large eyes with tiny pupils and two vampire-like teeth. Its appearance is likely based on the Japanese concept of hitodama.

Gastly is made of a very thin, gas-like substance, which grants it certain powers and weaknesses. Some say it is an other-dimensional lifeform, while others believe it is formed from toxic gas. It can sneak into any place if it so wishes thanks to the invasive properties of gas molecules. It can render itself nearly invisible and approach enemies undetected, and can envelop said enemies regardless of their size and induce sleepiness, slow poisoning through the skin, or even suffocation.

On the other hand, Gastly's negligible weight and gaseous nature make it very easy for it to be blown off or even scattered to nothingness under a strong wind (although it is presumed the molecules can regroup later). A Gastly will not appear in windy weather if it can help it, but when it finds itself threatened by a strong wind, it clusters in groups under the eaves of houses to protect itself.

Gastly is said to frequent decrepit, deserted buildings.