Lucario and Ditto are on a team in a double battle. Lucario mega-evolves and Ditto uses transform straight after. Would he transform into Lucario or mega-Lucario?
Ditto can Transform into Mega Evolution's and maintain the form, regardless of whether it holds the required item (Mega Stone) or not.
Ditto will copy a Mega Evolution's Appearance, Ability, Weight, Type, Stat Changes, Moves and Stats Excluding HP. It has the same effect on Mega Evolution's as any other Pokémon.
Note: It will not copy an opponent's IV's or Gender.
Hope I helped :)
He would transform into Mega Lucario.