So in the Beta footage, at [url=http://youtu.be/TljtC5qnb-s?t=1m56s]1:56[/url] there is a shot of something guardians are fighting, it happens again at [url=http://youtu.be/TljtC5qnb-s?t=2m5s]2:05[/url].
Upon further analysis and discussion (Thanks Clutchmaster, ghost59, and ghostdiver) it's looking more likely that this is a Vex hydra, a not yet seen aerial vex unit apart from harpies. And as it looks like a machine, and nothing fallen like, I'm assuming it's Vex. The fact that their are 2 players fighting it only also kind've indicates that this probably isnt anything as serious as a Raid.
Edit: Ghost59 brought up a good point. It seems this could very well be a glimpse at a Vex Hydra. It's fricking huge, I'm so stoked.
[spoiler]Anyone else getting a Juggernaut from Marathon Vibe?[/spoiler]
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Here are the small ones