Well, a recent Destiny interview with Dave Dunn sheds some light on the mysterious beast:

Dave: [Laughs] No, but there was a point where I tried to -- this is always a funny story at our studio -- I did the mission in Halo 2, which was called The Alpha Gas Giant, which was this floating mining facility, and I was, like, “we need to have something in the atmosphere”. I came up with the idea where I tried to get these space jellyfish in there. I remember I was going to record the sounds that were, like, “bloop bloop”, and at some point -- we were working really late at this point -- somebody with a clearer mind said to me “That’s the stupidest looking thing I’ve ever seen, you’ve got to take that out of there”, and I was, like, “Yeah, you’re probably right”.