With today's leak of Japanese magazine CoroCoro, Pokemon fans were introduced to three new Mega Evolutions as Salamence, Altaria and Lopunny recieved new forms.
Salamence will retain its Dragon/Flying type when in its Mega form. Mega Salamence has quite an interesting design, looking like it is meant to to resemble some sort of glider. While a bit odd, it is most definitely a pretty original look.
Mega Salamence will gain the ability Aerilate, which changes Normal-type moves into Flying-type moves. Being part Flying, this will allow Salamence to gain an attack bonus when using Normal-type moves.
Lopunny will become a Normal/Fighting type upon Mega Evolution as opposed to just being a Normal-type. The main design change between it and regular Lopunny is centered around its ears, which become substantially thinner.
As for its ability, Mega Lopunny gets a huge buff by gaining Scrappy. Scrappy allows both Normal and Fighting-type moves to hit Ghost-type Pokemon, giving Mega Lopunny a big advantage in battle.
Altaria probably got the biggest power boost of today's Mega forms, as its Dragon/Flying typing was changed to Dragon/Fairy. This new typing makes it complete immune to Dragon-type attacks, which were once super-effective against Altaria. Its design doesn't change too drastically, just adding a few more clouds around it.
Much like Mega Salamence, Mega Altaria will get an ability that changes Normal-type moves. With the Pixelate ability, all Normal-type moves used by Mega Altaria will be changed to the Fairy-type.