If you mean Gengar (Fantina the 5th gym leader has one ) You have to evovle Haunter by trading it.
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Yes, but you must trade a Haunter with a friend to get it to evolve
Pokemon diamond
you cant be a Pokemon in Pokemon diamond
To get a Gengar you must first catch a Haunter. To evolve the Haunter you have to trade it to another gaming system.
Pokemon 134 is Finneon in diamond