Thanks to meerkat313 for suggesting this mega!
Mega Butterfree remains a bug and flying type. It gets shield dust for its ability, removing harmful effects from the opponents moves. Mega Butterfree gets defensive stat boosts and a speed boost, but its biggest stat gain is in sp attack. Mega Butterfrees appearance changes to where its wings become bigger and dust falls around the pokemon in a veil like fashion. Its body gets more powerful and larger.
Stat boosts:
20 defense
40 sp attack
25 sp defense
15 speed