Finding a Gengar in pretty simple in you insert any GBA game into your DS and go into The Old Chateau near Eterna City and go into the room with the eyes on the wall.
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A Gengar can be seen in a battle with Psychic Bryce in Victory Road.
u can not see him u have to trade a haunter to get a Gengar or a trainer will have one
you have to see all the Pokemon in the pokedeks
Oh, but not every Pokemon is cute. Especially the "uber" and legendary Pokemon, who actually look quite ferocious, and not so much cute. And some are pretty scary (see: Gengar, Spiritomb).
get it from a honey tree