Shadow over Hollin

During the Quest of the Ring, a mysterious shadow passed over the Fellowship of the Ring in the night, darkening the stars as it moved over Hollin. The origin of this shadow is unclear, as it was moving fast, and against the wind.
= Background =
Fans have developed a number of theories for the origin of the Shadow. Some believe that the shadow over Hollin was a Winged Nazgûl. However, this can be disproven; Grishnakh states that they weren't allowed to cross over the Anduin until the later stages of the War of the Ring. Also, the shadow passes over the Fellowship in early drafts of the story, even before the winged steeds of the Nine were conceived.
Another possibility is that the shadow was simply a flock of Crebain, which the Fellowship had seen earlier on. Alternatively, the shadow could have been a manifestation of Sauron's power; a similar shadow passes over Amon Hen later on when Frodo puts on the One Ring.