You can trade anything.
Basically, what happens is...
You trade a haunter, and it evolves as soon as it gets to the other side.
Then ask your friend to trade it back to you
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If you just trade the haunter with no items it will evolve to gengar now take it back so that you have the gengar. You can't evolve a haunter by migrating it.
Trade Haunter and you will get a Gengar.
To get a Gengar, you must trade a Haunter and it will then evolve into a Gengar.
There is no NPC who you can trade a Haunter to for a Gengar in Pokemon Crystal. To get Gengar you will need to trade with another player or a copy of Pokemon Gold, Silver or Crystal.
Yes. The pokemon is Haunter. In order to evolve Haunter you will have to trade him with a friend and he will evolve. Just trade back to get Gengar.