you get it over a trade.
You trade haunter to get gengar.
Haunter evolves when traded
You can't find gengar in the wild, you have to get a haunter and trade it.
Some lady in a city will trade you it but it will stay as haunter.
Haunter evolves only by trading to another game.
Actually, Haunter does evolve. Its evolves into Gengar when you trade it.
gastly, haunter, gengar
Nothing, to evolve haunter you have to trade him...
you can't find haunter in Black
trade
turn back cave
25 into Haunter and then if u trade Haunter he evolves into Gengar