I was doing Single Battles in the Battle Chateau in ORAS. I was fighting the Battle Chatelaine. She threw out her Purugly and I threw out my Sableye. I Mega Evolved my Sableye and used Will-o-Wisp. To my great surprise, he outsped Purugly – which is impossible with base 50 Speed and no investment – and Will-o-Wisp went first. I know that it takes a turn for Mega Pokémon's Speed stat to change, but does that include priority from Prankster, an Ability Sableye had before it Mega Evolved and gained Magic Bounce? Am I the only one to discover this???
They keep there ability first turn so mega mawile will have indimidate when it megas, much like sableye with prankster on the first turn
A Pokemon keeps its original ability the turn it Mega Evolves.