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 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 236/300/238/344/135.
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.
At Level 100 with no EV training, Hoopa's stats can be 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 is related to the Lumiose City blackouts.
At Level 100 with no EV training, Volcanion's stats can be 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 exactly 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.