The complete Pokemon RBY pokedex, part 9

Pokémon Name: Haunter
Type: Ghost/Poison
Classification: Gas Pokemon
Pokédex Number: 093
Ability: Levitate
Useful Attacks: Destiny Bond
Location Found:
Diamond/Pearl: Turnback Cave, Old Chateau (with R/S/E/Fr/Lg inserted)
Platinum: Turnback Cave
HG/SS: Route 8, Safari Zone (Forest)
Black: Trade, Poke Transfer
White: Evolve Gastly

Haunter, the second stage in this evolutionary chain, learns some seriously cool moves. Destiny Bond KOs the enemy if Haunter goes down, Nightmare slowly saps the life out of sleeping foes and Shadow Ball puts on the hurt while simultaneously lowering special defense. He's so top-shelf that we hardly want to evolve him further, but it must be done - Gengar has even higher stats.

Evolution: To Gengar via Trading