Gengar did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Gengar is a dual Ghost/Poison-type Pokémon species. It has the same abilities in battle as a Haunter, only stronger. The presence of a Gengar can cool the temperature of the surrounding area by 10 °F. They evolve from Haunter if it is traded to another player, who evolve from Gastly. They are a Generation I Pokémon. Gengar are dark purple, Pokémon with a roundish body. Gengar's hands now connect to the rest of its body. In addition, it now has stubby legs and with darker purple soles. And a short, pointed tail. Gengar has a spiky fur-covered back. Gengar's eyes are now red, and its mouth is a teethy sinister grin. His shiny form only changes his body color and make it darker.
It can Mega Evolve into Mega Gengar when using the Gengarite.