Gengar is a circle-shaped Pokémon with two large ears and spikes all over its back. Gengar has two red eyes and a sinister smile. It has claws on its hands. Gengar stands on its two legs, but it can Levitate as well.
Gengars have the ability Levitate. Levitate makes Gengar immune to Ground-type moves. This Pokémon hides in the shadows. It is said that if Gengar is hiding, it cools the area by nearly 10°F. When there is a Full Moon, this Pokémon likes to scare people by pretending to be their shadow, then laugh at their fear. Gengars are mostly based on being mean and evil by tormenting people, hurting them and overtaking them. It can absorb any surrounding heat, leaving a sudden chill. To steal the life of its target, it slips into the prey's shadow and silently waits for an opportunity. Gengar is also able to merge with and inhabit the shadow of another being.
Gengar is the evolved form of Haunter, by trade. However, it evolved into second final form, Mega Gengar.
Unlike most Pokémon, a Shiny Gengar has a small physical difference from a normal Gengar in HeartGold and SoulSilver. In said games, the spikes on a Shiny Gengar's back are shaped differently than a regular Gengar's.
Gengar is four inches shorter than Haunter, despite it being evolved from Haunter.
Mega Gengar, like Mega Alakazam, gets shorter when it mega evolves.
They are also the only Poison types to have a weakness to Dark-type attacks. In Generation 1 Ghost types were strong against Psychic types in theory only but due to Gengar and it's pre-evolutions (all of which are dual Poison type) are the only Ghost types in those games the reverse was true, Gengar and its pre-evolutions are the only Ghost types to have a weakness to Psychic-type attacks.
This is because no other Pokémon have the same type combination as Gengar and its pre-evolutions.
Gengar may have been based on the Oni, a creature of ancient Japanese folklore that was said to float invisibly above cities, bringing anything from a natural disaster like an earthquake to a terrible disease.
There is a theory stating that Gengar might be the shadow of Clefairy or Clefable.
This is because they have a similar physical appearance, because Gengar's name originates from the term "doppelganger," and because Gengar's species is Shadow Pokémon.
Gengar's Mega Stone is banned by Smogon on online battles in X/Y due to Shadow Tag and Perish Song.
Gengar appears to be based off a ghostly cat. Its name is possibly based off "doppelganger".