The Pokemon Company and Nintendo have shared the first gameplay glimpse of Pokemon Omega Ruby / Alpha Sapphire, the upcoming "full remakes" of Pokemon Ruby / Sapphire. According to a
Like a normal evolution, Mega Evolutions grant a pokemon extra power and change its physical appearance. However, Mega Evolutions are temporary, and only last for a single battle. The Mega Evolution forms of pokemon who can utilize them are also not counted in the franchise's expansive pokedex.
We'll update as more information becomes available.
Update: We've replaced the Serebii screengrab with a (very) brief trailer showing off the monsters in motion.