Here is information on all the Mega Pokemon which have been revealed for the upcoming Pokemon Alpha Sapphire and Omega Ruby. Some of the information, especially the base stats, were reported from hackers using demos of the upcoming Pokemon games, and are unconfirmed. The base stats represent hit points, attack, defense, special attack, special defense, and speed, in that order. Pokemon that receive a new type after mega evolving have that type bolded. Kyogre and Groudon obtain their new forms through primal reversion, not mega evolution.
Hoopa may be a form change triggered by a key item (like Shaymin/Deoxys), not a mega.
According to twitter user Kurt. Unlike Mega evolution, Primal Reversion is triggered immediately upon switching in. Although Stealth Rocks damage will be applied before the Primal Reversion (meaning Groudon takes slightly less Stealth Rocks damage). It isn't confirmed if you can have multiple Primal Reversions on the same team or not.
Image Species Type Ability Height Weight Base stats Altaria Dragon/Fairy Pixilate 4'11 45.4lb 75/110/110/110/105/80 Audino Normal/Fairy Healer 4'11 70.55lb 103/60/126/80/126/50 Beedrill Bug/Poison Adaptability 4'06 89.28lb 65/150/40/15/80/145 Camerupt Fire/Ground Sheer Force 8'02 706.58lb 70/120/100/145/105/20 Diancie Rock/Fairy Magic Bounce 3'07 61.3lb 50/160/110/160/110/110 Gallade Psychic/Fighting Inner Focus 5'02 124.37lb 68/165/95/65/115/110 Glalie Ice Refrigerate ??? ??? 80/120/80/120/80/100 Groudon Ground/Fire Desolate Land 16'05 2203.9lb 100/180/160/150/90/90 Hoopa Psychic/Dark Magician ??? ??? 80/160/60/170/130/80 Kyogre Water Primordial Sea 32'02 948.00lb 100/150/90/180/160/90 Latias Dragon/Fairy Multiscale ??? ??? 80/80/110/110/180/140 Latios Dragon/Psychic Adaptability ??? ??? 80/110/80/180/110/140 Lopunny Normal/Fighting Scrappy 4'03 62.4lb 65/136/94/54/96/135 Metagross Steel/Psychic Tough Claws 8'02 2,078.7lb 80/145/150/105/110/110 Rayquaza Dragon/Flying Delta Stream ??? 864.12lb 105/180/100/180/100/115 Sableye Dark/Ghost Magic Bounce 1'08 354.9lb 50/85/125/85/115/20 Salamence Dragon/Flying Aerilate 5'11 248lb 95/145/130/120/90/120 Sceptile Grass/Dragon Lightning Rod 6'03 121.7lb 70/110/75/145/85/145 Sharpedo Water/Dark Strong Jaw 8'02 287.26lb 70/140/70/110/65/105 Slowbro Water/Psychic Shell Armor 6'07 265.55lb 95/75/180/130/80/30 Steelix Steel/Ground Sand Force ??? ??? 75/125/230/55/95/30 Swampert Water/Ground Swift Swim 6'03 224.9lb 100/150/110/95/110/70