During a live gameplay demo of Halo 4, it appears that a yet-to-be-revealed enemy, believed to be some sort of indigenous species, was shown.
At the three minute mark, GameSpot's presenter spotted the mysterious enemy - presumably some sort of animal - floating above protagonist Master Chief, who was being played by a 343 Industry employee showcasing the demo. He was, however, quick to disregard the sighting, further adding to the credence of a new enemy type.
Halo 4's E3 demo revealed another enemy entitled the Promethean Knight. The game itself is currently due for a worldwide launch on November 6th, 2012.