Ludicom

Diancie is Rock/Fairy, 2'04", 19.4 lbs. #151. It is known as the Jewel Pokemon. "It can instantly create many diamonds by compressing the carbon in the air between its hands."
Diancie's signature move is Diamond Storm, a physical Rock-type attack. A bunch of diamonds are created and thrown at the enemy. "The user whips up a storm of diamonds to damage opposing Pokemon. This may also raise the user's Defense stat." It has 100 power and 95% accuracy. Diancie can also learn Moonblast, Safeguard, and Light Screen.
Diancie is not related to Carbink on its Pokedex evolution page. However it could be a Manaphy/Phione situation. Or maybe the diamonds it throws turn into Carbink after a couple million years? :p
At Level 100 with no EV training, Diancie's stats can be 222/236/300/236/344/135.
Hoopa in Pokemon X and YHoopa is Psychic/Ghost, 1'08", 19.8 lbs. #152. It is known as the Mischief Pokemon. "It gathers things it likes and pushes them through its loop to teleport them to a secret place."
Hoopa's signature move is Hyperspace Hole, a special Psychic-type attack. A dark hole is created in the ground, Hoopa goes through it, then comes out behind the enemy and hits them. "Using a hyperspace hole, the user appears next to the target and strikes. This also hits a target using Protect or Detect." It's got 80 power and N/A accuracy. Hoopa can also learn Psychic, Shadow Ball, and Trick Room
At Level 100 with no EV training, Hoopa's stats can be 301/231/154/315/296/177.
Volcanion in Pokemon X and YVolcanion is Fire/Water, 5'07", 429.9 lbs. #153. "It expels its internal steam from the arms on its back. It has enough power to blow down a mountain."
Volcanion's signature move is Steam Eruption, a special Water-type attack. "The user immerses the target in superheated steam. This may also leave the target with a burn." It's got 110 power and 95 accuracy. Volcanion can also learn Flare Blitz, Overheat, and Explosion.
Volcanion is related to the Lumiose City blackouts.
At Level 100 with no EV training, Volcanion's stats can be 275/254/276/247/235/176.
Mega Latios and Latias in Pokemon X and YThere are no other Mega Evolutions in the game aside from Mega Latios and Mega Latias, which look the same. They both retain their original Dragon/Psychic typing.
Celebi remains Psychic/Grass.
The extra meteor for Deoxys at the Fossil Lab is a Red Herring. It's just another attack forme meteor.