[Spoilers after the jump]
I'm sure like me, your first thought was that this was a totally new addition to the Halo universe. However, after a little bit of thinking I remembered these guys:
So far as I'm aware they're unnamed creatures that only ever appeared in the E3 1999 Halo demo.
There's certainly a very close resemblance between the creatures from '99 and the Crawlers we see today. Not identical, but then nothing in the E3 '99 demo made into the games without substantial changes. It's just my theory, but it'd be really cool if 343 were pulling in bits of long lost Halo lore like this.
- Cowboy Out.