Mega Charizard X (メガリザードンX, Mega Lizardon X) is the Mega Evolution of Charizard in the Pokémon games using a Mega Stone that is exclusive to Pokémon X. It appears in Super Smash Bros. 4. It is currently unknown how the Mega Evolution is activated, nor how much stronger Charizard gets when in this form.
Origin
Mega Evolution is a mechanic introduced in Pokémon X and Y. Only a small amount of Pokémon—including Charizard—can Mega Evolve, and to do so it must hold a special Mega Stone unique to its species. Charizard Mega Evolves into Charizardite X and Charizardite Y. Charizard is one of the few Pokémon that have two Mega Evolution forms. Certain Pokémon have two Mega Evolution forms, an "X" form and a "Y" form, with the particular Mega Stone only appearing in Pokémon X and Pokémon Y respectively.
Gallery
Mega Charizard X flying in Super Smash Bros. for Wii U.
Close up of Mega Charizard X in Super Smash Bros. for Wii U.