Ultra-sun: You can hear tales told all over the world about how Gengar will pay a visit to children who are naughty.Moon: It apparently wishes for a traveling companion. Since it was once human itself, it tries to create one by taking the lives of other humans.Alpha-sapphire: Sometimes, on a dark night, your shadow thrown by a streetlight will suddenly and startlingly overtake you. It is actually a Gengar running past you, pretending to be your shadow.
Prof. Lotus' Notes: Gengar's name origin possibly comes from the German word, Doppelgänger. The longstanding theory is Gengar is Clefable's shadow, they share many physical traits, that with the addition of Fairy type makes Gengar strong against Clefable. The even darker theory is Gengar is the ghost of dead Clefable.