The complete Pokemon RBY pokedex, part 9

Pokémon Name: Gastly
Type: Ghost/Poison
Classification: Gas Pokemon
Pokédex Number: 092
Ability: Levitate
Useful Attacks: Sucker Punch
Location Found:
D/P/P: Lost Tower, Old Chateau
HG/SS: Sprout Tower, Bell Tower, Safari Zone (Forest); Pokewalker - Dim Cave, Scary Cave
Black: Trade, Poke Transfer
White: White Forest (resident required: Dave)

One of the most famous Ghost types out there, Gastly has three highly useful stages to consider. Their high special attack stats make moves like Shadow Ball a real threat. Don't discount Sucker Punch though - it's "first to fight" quickness ensures an immediate strike if your opponent is using an attack move.

Evolution: To Haunter at 25