post #444849 post #628523 post #403864

Source:
https://bulbapedia.bulbagarden.net/wiki/Gengar

As Mega Gengar, a portion of its body sinks into the ground and its arms and tail lengthen. The lower portions turn a more reddish-purple, and a third eye appears on its forehead. This new unblinking eye is oval, yellow, and allows it to see into other dimensions.The spikes on Mega Gengar's back are more numerous and pointed, and additional spikes appear on its hands and tail. Its ears are now considerably longer and tilt backward.

Mega Gengar has the ability to access other dimensions and has a vicious nature. Mega Gengar has no interest in opponents unless perceived as prey. Its relationships are warped due to Mega Evolution. It now desires to take and curse the life of anyone or anything in its path; it may even curse its own Trainer. Mega Gengar sinks into another dimension to wait for a chance to attack.

Shiny mega gengar is white instead of dark purple.

Until now,no other Pokémon has the same type combination as Gengar and its pre-evolved forms.
(The above content is from the bulbapedia.)

See Also:

• pokémon_(species)
• gastly
• haunter
• gengar
• mega_gengar
• gigantamax_gengar