This is the troll mega evolution.
Ability: Mold Breaker
Thunder Wave- Even though Mold Breaker was added for maximum sweeping potential, it turns Gyarados into less of one. Now Thunder Wave can paralyze anything.
Taunt- Combined with Thunder Wave this is a real troll.
Waterfall- STAB that pretty much OHKOs everything.
Earthquake- Another OHKO move.
Mega Gyarados is a different Pokemon than what he once was and why Game Freak didn't change him into a Water Dragon type is beyond me. Give him a Jolly nature and 252 HP/128 Atk/128 Special EVs.