Meh, this is my first time trying to think up a Mega and drawing it.
Mega Trevenant is a Grass/Ghost type. It's the opposite of Mega Gengar, slow and tanky. Plus being able to hit like a truck, physically of course.
It's ability would probably be Grass Pelt, being able to gain even more Defense in Grassy Terrain.
The Stats would boost each Defense and Special Defense by 40 points, take 10 points away from Speed and add 30 points to Attack.