Mega Haxorus would get the (made up) ability Onslaught, which raises the pokemons speed and attack when it knocks out a foe. Its attack and speed greatly rise and its sp defense rises slightly. Its appearance changes to where its tusks are bigger and sharper. It's stronger and bigger and has lighter but slightly more durable armor. It gets red streaks along its body. It also gets curved spikes on its body and its claws are longer looking.
Stat boosts:
40 speed
50 attack
10 sp defense