It's a person in the bottom, a person in a suit.
The same person that burns someone/thing in the leaked pics from SDCC.
They are in the vehicules hangar (check the lights).
I would be very suprised that the "thing" jumping on the crew member is an actual creature coz from the shot it makes no doubt that you will be able to see it clearly in the HD trailer, spoiling one of the major asset very early seems unlikely.
My money is on a crew member (an artificial human maybe) gone rogue...