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.
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if you are talking about Pokemon you simply trade it to another persons game and it will evolve once its traded you can trade back after and you will have a gengar same applies for alakazam and various other Pokemon too.(some Pokemon are required to be traded while holding a specific item before they will evolve like onix to steelix where onix need to be holding a metal coat)
From the top of my head, I can think of graveler, kadabra, machoke, haunter, onix(need item), scyther(needs item), clamperl(needs deepseatooth or deepseascale), porygon(needs item), porygon2(needs item), magmar(needs item), electabuzz(needs item), I think there are more, but that's all I can think of.
When looking at the item, it does say "a Pokemon loves this item". It is a hint to give a Pokemon that item. Well that Pokemon is a rhydon. Give a rhydon that item to hold. Trade it and see what happens...
Of course, how else would you get sizor and steelix. just give the Pokemon an item and trade!
get an onix, then give it the item metal coat, and trade it to another game. it should evolve