Pokemon X and Y will pit your one Pokemon against five in certain wild encounters, and will take some fights to the skies, Junichi Masuda, Director of Game Freak, announced during an E3 2013 developer roundtable today.
Horde Battles will occur at certain points in the wild when encountering Pokemon in the traditional manner. Instead of fighting just one, you'll have to fight off five at a time. The risks are obviously high, but if you can take them all out — a process made easier with attacks which target multiple Pokemon at a time — you'll manage to earn a lot more experience for your single Pokemon.
Sky Battles will be brought upon by certain trainers, and can only be participated in by certain Pokemon, like Flying-type, and some Ghost and Dragon-type creatures. These battles take place in the sky, and feature more cinematic battle camera angles, as the player's perspective swings around the two dogfighting flyers. Any mechanical differences present in this special battle mode, save for the restriction on participating Pokemon types, weren't detailed during the presentation.