I'm not sure how accurate this is but...
First to call that the difference in Mewtwo forms to be a male and female split!
TALKING BOUT THEM X AND Y CHROMOSOMES
(Melee Mewtwo returning with this form is fully justified now)
Here it is
Fairy will be a new type. It is weak to Poison and Steel, immune to Dragon, super effective against Dragon, Dark, and Fighting, and Fire and Psychic-type Pokemon take half damage from it.
Sylveon is Fairy-type. Some older Pokemon will be reclassified to Fairy, such as Mawile (Steel/Fairy).
Mewtwo has two new formes, not just one. Both are activated via items.
The professor's English name is Patrice.
The Starter final evolutions are Fire/Psychic, Grass/Dark, and Water/Fighting.
The first Gym is Bug, the second is Fairy, and the fourth is Fighting.
A few attacks will be dual-typed.
There will be a new type of battling style that Flying-type Pokemon and Pokemon with Levitate can participate in.
You will be able to see a Pokemon's EVs at a certain in-game facility. You will be able to boost the EVs through mini-games. It will take about two hours to get one stat to max EVs.
Pancham evolves into Pangoro in the early level 30s. In order to evolve Pancham into Pangoro, you need to have one other Dark-type Pokemon in your party. Pangoro's type is Fighting/Dark.(Note, Evolution method is said to be new, but not confirmed)
A Pokemon named Inkay evolves into Malimar. To evolve it, you have to hold your 3DS upside-down when it levels up (the 3DS has an accelerometer and gyroscope, so it can detect that). Malimar's type is Dark/Psychic.(This Pokémon line is based around flipping upsidedown, by all means, pretend that isn't basically confirmed)
A Psychic-type Pokemon named Espurr (ESP + purr) evolves into Meowstick (Meow + mystic?).
Like in B/W you have two rivals(There are no less than 4 other characters from the starting town, however none of them have been stated to be rivals) - one is friendly and the other is competitive. Your rival that is more friendly gives you his/her fully evolved starter Pokemon after you beat the game.
Another professor named Sycamore gives you either Bulbasaur, Charmander, or Squirtle.
Even though it shows the player riding on a Gogoat in the new trailer, players can't ride on their own Pokemon. What was shown is just an alternate way of getting around Lumiose City and it only works with Gogoat.(Rhyhorn is confirmed. However he could have meant 'Only Gogoat in Lumiose')
HMs still exist but there's a lot less of them.
Players will be able to see their Pokemon's EVs (probably through the same in-game facility mentioned in the other post)
A new move, Belch, is a Poison-type attack that has a base power of 120. The user must consume a held Berry to perform it.
Draining Kiss is a Fairy-type attack that heals the user for the same amount of damage performed. It has a base power of 60.
If the previously revealed forme of Mewtwo can be said to look roughly similar to Mewtwo's original design, the second supposed form of Mewtwo can be said to look roughly/vaguely similar to Mew's design.
That is the leak. About 70-80% of it has been proven to be true, nothing on that leak was confirmed at the moment it was posted and it was only posted because the leaker sent a previous email guessing the names of 3-5 of the new pokemons (which further increases the leaks validity).
Edit: If you are going to validate this leak try not to get confused with Corocoro's recent leaks which has basically the same information in this leak but with different names. The reason for that is because Corocoro is leaking the japanese version of the game on which the names of the Pokemons, cities and proffesors are different.
If anything the fact that Corocoro is just recently leaking information for the japanese version that was already leaked in this post for the english version back in March goes to tell you just how much this leaker actually knew!
Edit2: I don't know where I got that the names of the forms would be Mewtwo X and Mewtwo Y, it is not in the leak apparently.
The following is my own speculation and not part of the leak:
I believe that by item he means that you will obtain an item that will make Mewtwo transform as long as he is holding it (Think of Arceus plates changing his type.)
I believe the items will probably be version exclusives, giving each version its own Mewtwo form.
Pokemon Z will probably either give you both items or give you a new item all together with a 3rd Mewtwo form!
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But most importantly
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In the game there is no new Mewtwo apparently, or at least it hasn't been hinted as that there will be. If there is no new Mewtwo in the game and you just transform the old Kanto Mewtwo then this gives me hope for his old Melee personality to come back! :D