They are too small to make out in detail, and they move too regularly to be errors in the game or compression artifacts. The most popular interpretation at this point is that these are some sort of insect native to the Halo. During the E3 movie, a jeep drives through a herd of these small creatures. They behave as a pack, avoiding the jeep and snarling at it as it goes by. This appeared in a Halo article in PC Gamer UK magazine.
Nothing is current ly known about it.
They seem to be heavily armored Covenant warr iors, perhaps Halo's equivalent of Marathon's Hunters. This alien was first spotted in a screenshot at E3 2001. It wears armor similar to an Elite, but it has a very different head. It is also equipped with a personal forcefield.