US Name: Mega Venusaur
Jp. Name: Mega Fushigibana

Type: Grass/Poison
Classification: Seed Pokémon
Height: 7' 10"
Weight: 342.8 lbs.
Ability: Thick Fat

First Appearance: Pokémon X & Y

Details:
This Pokémon was first revealed in the Pokémon Direct on September 4th. This Pokémon is to Mega Evolve from the Bulbasaur you receive from Professor Sycamore in Lumiose City. It has increased Defense and Special Defense stats.

US Name: Mega Charizard X
Jp. Name: Mega Lizardon X

Type: Fire/Dragon
Classification: Flame Pokémon
Height: 5'07"
Weight: 243.6lbs
Ability: Tough Claws

First Appearance: Pokémon X & Y

Details:
This Pokémon was revealed during the airing of the special episode Pokémon Origins. This Mega Evolution is exclusive to the game Pokémon X and has an increased Attack stat

US Name: Mega Charizard Y
Jp. Name: Mega Lizardon Y

Type: Fire/Flying
Classification: Flame Pokémon
Height: 5' 07"
Weight: 221.6 lbs.
Ability: Drought

First Appearance: Pokémon X & Y

Details:
This Pokémon was first revealed in the Pokémon Direct on September 4th. This Pokémon is to Mega Evolve from the Charmander you receive from Professor Sycamore in Lumiose City. It has a significantly increased Special Attack stat. It was later revealed to be exclusive to Pokémon Y

US Name: Mega Blastoise
Jp. Name: Mega Kamex

Type: Water
Classification: Shellfish Pokémon
Height: 5' 03"
Weight: 222.9 lbs.
Ability: Mega Launcher

First Appearance: Pokémon X & Y

Details:
This Pokémon was first revealed in the Pokémon Direct on September 4th. This Pokémon is to Mega Evolve from the Squirtle you receive from Professor Sycamore in Lumiose City

US Name: Mega Mewtwo X
Jp. Name: Mega Mewtwo X

Type: Psychic/Fighting
Classification: Genetic Pokémon
Height: 2.3m
Weight: 127.0kg
Ability: Steadfast

First Appearance: Pokémon X & Y

Details:
This Pokémon was revealed within the October issue of CoroCoro in September 2013. It is a Mewtwo Evolution that is exclusive to Pokémon X and has a different type to its counterpart in Pokémon Y. It has an increased Attack stat

Item Required
Mewtwonite X

US Name: Mega Mewtwo Y
Jp. Name: Mega Mewtwo Y

Type: Psychic
Classification: Genetic Pokémon
Height: 1.5m
Weight: 33.0kg
Ability: Insomnia

First Appearance: Extremespeed Genesect: Mewtwo Awakens

Details:
This Pokémon was first leaked in a promotional pamphlet of the Present Guide for the movie Extremespeed Genesect: Mewtwo Awakens, replacing Mewtwo on the poster with a caption calling it the Eclair Forme. However, soon afterwards it was revealed by Pokémon Smash, though without saying it is actually a Mewtwo form with the official sites stating it as a new Pokémon.
It was eventually confirmed as a Mega Evolution in CoroCoro in August 2013 and was revealed to be exclusive to Pokémon Y in September 2013. It has an increased Special Attack stat

Item Required
Mewtwonite Y

US Name: Mega Blaziken
Jp. Name: Mega Bashamo

Type: Fire/Fighting
Classification: Blaze Pokémon
Height: 1.9m
Weight: 52.0kg
Ability: Speed Boost

First Appearance: Pokémon X & Y

Details:
This Mega Evolution was first shown in August 2013's issue of CoroCoro. It features the special ability of Speed Boost and has an increased Attack stat

Item Required
Blazikenite

US Name: Mega Lucario
Jp. Name: Mega Lucario

Type: Fighting/Steel
Classification: Aura Pokémon
Height: 1.3m
Weight: 57.5kg
Ability: Adaptability

First Appearance: Pokémon X & Y

Details:
This Mega Evolution was first shown in August 2013's issue of CoroCoro. It features the special ability of Adaptability. It also has an increase Attack stat

Item Required
Lucarionite

US Name: Mega Mawile
Jp. Name: Mega Kucheat

Type: Steel/Fairy
Classification: Deceiver Pokémon
Height: 1.0m
Weight: 23.5kg
Ability: Huge Power

First Appearance: Pokémon X & Y

Details:
This Mega Evolution was first shown in August 2013's issue of CoroCoro. It features the special ability of Huge Power. It has increased Defense & Special Defense

US Name: Mega Absol
Jp. Name: Mega Absol

Type: Dark
Classification: Disaster Pokémon
Height: 1.2m
Weight: 49.0kg
Ability: Magic Bounce

First Appearance: Pokémon X & Y

Details:
This Mega Evolution was first shown in August 2013's issue of CoroCoro. It features the special ability of Magic Bounce. It has an increase Attack and Speed

US Name: Mega Ampharos
Jp. Name: Mega Denryuu

Type: Electric/Dragon
Classification: Light Pokémon
Height: 1.4m
Weight: 61.5kg
Ability: Mold Breaker

First Appearance: Pokémon X & Y

Details:
This Mega Evolution was first shown in August 2013's issue of CoroCoro. It features the special ability of Mold Breaker and features a change in type to Electric/Dragon

US Name: Mega Kangaskhan
Jp. Name: Mega Garura

Type: ??
Classification: Parent Pokémon
Height: ??
Weight: ??
Ability: Parental Bond

First Appearance: Pokémon X & Y

Details:
This Mega Evolution was first shown in the Video Game Championships in Vancouver in August 2013. It's unique in that is allows the baby in Kangaskhan's belly to come up and battle with the focal point of the evolution being based on the baby

US Name: Mega Garchomp
Jp. Name: Mega Gaburiasu

Type: Dragon/Ground
Classification: Mach Pokémon
Height: 1.9m
Weight: 95.0kg
Ability: Sand Force

First Appearance: Pokémon X & Y

Details:
This Pokémon was revealed within the October issue of CoroCoro in September 2013. It has decreased Speed but increased Attack and Special Attack