The other recently leaked Mega Evolution has officially been confirmed! Mega Slowbro was spotted early alongside Mega Audino—which got its own reveal trailer a couple days ago—and now we have a trailer that made its debut at the 2014 Pokémon World Championships.
Here's a description from the press release:
Slowbro is a Water- and Psychic-type Pokémon and one of two evolved forms of Slowpoke, the other being Slowking. When Slowbro Mega Evolves into Mega Slowbro, its energy gathers in the Shellder clamped onto its tail. As a result, the hardness of its shell is boosted, making its armor incredibly strong. In battle, Mega Slowbro balances on its tail, wearing Shellder as an impenetrable armor over its body.
Mega Slowbro has the Shell Armor Ability, meaning opponents can never land a critical hit, making Mega Slowbro a defensive threat in battle. Its Defense and Sp. Attack also increase, making it a great teammate for long battles requiring high endurance.
Pokémon Omega Ruby and Alpha Sapphire hit Nintendo 3DS on November 21.