Gengar

Gengar (ゲンガー Gangar) is a /-type Pokémon, and is the evolved form of Haunter.

Appearance
Gengar is a circle-shaped Pokemon 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 too.

Special Abilities
Gengars have the ability Levitate. Levitate makes Gengar immune to -type moves. This Pokémon hides in the shadows. It is said that if Gengar is hiding, it cools the area by nearly 10 degrees F. When there is a Full Moon, this Pokémon likes to scare people by pretending to be their shadow. Then, Gengar laughs at their fear. Gengars are mostly based on being mean and evil by tormenting people and 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.

Game/Apearances
In the original games, as well as the GBA games, Gengar can only be obtained through trading to another game. In Diamond & Pearl he appears in the Haunted Mansion, in a room with a painting with eyes that follow you around, very rarely, if one already has the National Pokédex and a Pokémon game inserted in the GBA slot of the DS. In the Rescue Team version he is a boss in Dungeon 4: Sinister Woods. Gengar appeared in the anime episode, "Night In A Haunted Tower!" and team up with Gastly and Haunter. But in the end, they became silly and happy.

TCG Pokédex Entry (Dark Gengar)
If a Pokémon was to disappear in darkness, it would certainly have to be the work of Gengar.

Trivia
Despite Gengar able to sneak in shadows, it is unable to learn Shadow Sneak.

Despite being part Poison type, it is only able to learn 2 Poison type moves.

Gengar is one of the two only stage two Ghost-Type Pokémon. The other being Dusknoir

Unlike most Pokemon, 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

Origin
It is based on some kind of Ghost Cat.