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: Shiny Mega Gengar is white. In Lavender Town, you meet a girl standing outside the haunted tower who asks you 'Do you believe in Ghosts?' then says 'Hahaha, I guess not. That white hand on your shoulder... it's not real'. If you look at Gengar's design, those hands... *Spooky*