It uses its docile-looking face to lull foes into complacency, then bites with its huge, relentless jaws.
– Pokemon X Pokedex
Mega Mawile’s jaw-hair turns into two jaws, and its hakama thing turns pink.
When it mega evolves, its Attack, Defense, and Sp Defense increases. Though its base stat total is still only 480 (lower than some normal fully evolved Pokemon). However, its ability is Huge Power, which doubles its Attack, making its base Attack 210. The downside is that its Speed didn’t increase, so it still has base 50 Speed. It does learn one priority move in Sucker Punch, that helps.