Gastly

Gastly is one of the only 3 Ghost type Pokemon of the Kanto region. As implied by its name, it's a gas entity. Despite it and its evolved forms being the only Ghost type Pokemon of the Kanto region, none of them are pure Ghost types. All 3 of them are actually dual Ghost/Poison types. A pure Ghost type was eventually introduced in the next region.

Gastly can evolve into a Haunter and then into a Gengar if traded away.

Its name is most likely a combination of gas and ghastly. Kind of a play on ghastly by adding gas to it since it's a gassed entity.

Its Japanese name ゴース Ghos comes from the English term ghost.

Gastly is largely composed of gaseous matter. When exposed to a strong wind, the gaseous body quickly dwindles away. Groups of this Pokémon cluster under the eaves of houses to escape the ravages of wind.

Gallery

A Gastly forming hands from its gas

Who's that Pokemon?

Who's that Pokemon? (X and Y edition)