In the new video ‘the magic behind Halo 4’ you can see what looks like a new enemy variant. At the 2.15 mark, pause it (its difficult I know) and to the left of the blue promethean knight (which is interesting in itself) you will see something on all fours, probably a crawler variant. And to the right of the knight at around about the same point is another enemy running on two legs, which looks similar in size to a grunt.
Other tidbits in the trailer include clear views of jackals, grunts, scorpions on a large battlescape, and someone (I can’t tell who) climbing into and driving the mech.