Gastly, the Gas Pokémon. Although it can sneak in anywhere with its gaseous body, it can also be blown away by wind.
– Dawn’s Pokedex
Gastly is the original Ghost Pokemon, it likes to show up in places like the Spirit Tower or the Burnt Tower, or any abandoned place, haunting it. It is also quite abundant in Morty’s gym.
Its type is Ghost/Poison (Gen I was in love with Poison or something XD), the only Ghost/Poison family. The Poison part is often neglected or forgotten I think. Poison doesn’t really offer much to Ghost in terms of type advantages; it’s resistant to Bug and Poison, which Ghost is already resistant to, and resistant to Fighting, but Ghost is already immune to Fighting. It adds weaknesses to Psychic and Ground, but luckily Gastly has Levitate to avoid Ground.
Gastly always looks mean, or at least mischievous. That reminds me of this awesome comic on Rare Candy Treatment.
Stat-wise, its Sp Attack and Speed are pretty high, especially for a first-stage evolution Pokemon, but it means its other stats are pretty low, including defenses and HP.