the following can mega evolve: houndoom, manetric, aerodactyl, gyarados, alakazam, abomasnow, pinsir, heracross, gardevoir, medicham, scizor, banette, tyranitar, aggron, gengar, kanghaskan, charizard, mewtwo, garchomp, venusaur, blastoise, ampharos, absol, mawile, lucario, blaziken, and that's it.
