Concept art revealed by Bungie seem to show several different groups of enemy:

• The insect-like Fallen with their humanoid shapes and insectoid eyes;
• Space Zombies (not the official name) presumably something like Halo's Flood;
• The Vex: time-traveling robots
• The rhino-like Cabal

...there’s something primordial and instantly terrifying about squaring off against an unpredictable Fallen Captain, his tattered crimson cape draped over a set of four splayed and angular arms that bristle with sleek firepower.

--Urk, Bungie Mail Sack
http://www.bungie.net/7_The-March-of-the-Mail-Sack/en-us/News/News?aid=1...

[The Vex] are unlike any enemies I’ve fought in other games. The fiction behind them is really fascinating, which, in turn, creates one of the coolest deaths in the game.

David Candland, Senior Artist
Bungie Mail Sack
http://www.bungie.net/7_The-March-of-the-Mail-Sack/en-us/News/News?aid=1...